Annual report pursuant to Section 13 and 15(d)

Merger and Acquisition Activity - Additional Information (Detail)

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Merger and Acquisition Activity - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 01, 2021
Jul. 21, 2021
Sep. 01, 2020
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Mar. 31, 2021
Business Acquisition Contingent Consideration [Line Items]              
Business acquisition, total consideration       $ 108,883,000 $ 717,961,000 $ 120,897,000  
Maximum contingent consideration obligation       129,200,000 102,400,000    
Total consideration transferred, net of cash acquired           $ 151,600  
Accounts Payable and Accrued Liabilities [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination contingent consideration fair value current       14,400,000 5,500,000    
Other Non-Current Liabilities [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination contingent consideration fair value non current       27,600,000 16,600,000    
Employee Benefits [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities     $ 8,000,000.0        
Onex [Member] | Parent Company [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination assets acquired   $ 83,500,000          
Business combination consideration net of cash acquired   $ 343,200,000          
Onex [Member] | Parent Company [Member] | Common Stock [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Stock issued upon conversion   14,617,675          
Onex [Member] | Redeemable Preferred Units [Member] | Parent Company [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Number of shares acquired during period   260,000,000          
Onex [Member] | Common Class A [Member] | Parent Company [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Stock issued during period shares, Conversion of units   20,680,420          
Onex [Member] | Common Class X [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Common stock repurchase and retirement   640,784          
Ryan Re [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business acquisition, percentage of voting interests acquired             53.00%
Impairments of acquired goodwill and other intangible assets       0      
Crouse [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business acquisition, total consideration $ 110,600,000            
Keystone [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination assets acquired       $ 59,800,000      
ARL [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business acquisition, total consideration     814,900,000        
Business combination consideration net of cash acquired     40,800,000        
Total consideration transferred, net of cash acquired     1,223,300,000        
Business acquisition, equity interest issued or Issuable, value     102,000,000.0        
ARL [Member] | Long Term Incentive Plan [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities     $ 257,600,000        
Socius Insurance Services [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination assets acquired         1,300,000    
JEM Underwriting Managers, LLC [Member]              
Business Acquisition Contingent Consideration [Line Items]              
Business combination consideration net of cash acquired         $ 4,000,000.0