Schedule I - Summary of Investments Other Than Investments in Related Parties (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	$ 10,416,611
Fair Value	10,748,664
Amount at which shown in the balance sheet	10,748,664
Equity investments	729,721	$ 367,125
Fair value of other investments in funds or companies owned by or affiliated with related parties	2,524,420	1,957,757
Stone Point
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fixed maturity investments	15,100
Affiliated entity
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value of other investments in funds or companies owned by or affiliated with related parties	1,741,400
Affiliated entity | Trident
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Equity investments	96,800
AmTrust Financial Services, Inc.
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Indirect investment	240,100
Equities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	312,407
Fair Value	367,812
Amount at which shown in the balance sheet	367,812
Other investments, at fair value
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	783,032
Fair Value	783,032
Amount at which shown in the balance sheet	783,032
Funds Held - Directly Managed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	7,284,703
Fair Value	7,562,076
Amount at which shown in the balance sheet	7,562,076
Funds Held - Directly Managed | U.S. government and agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	319,554
Fair Value	325,731
Amount at which shown in the balance sheet	325,731
Funds Held - Directly Managed | U.K. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	140,194
Fair Value	146,423
Amount at which shown in the balance sheet	146,423
Funds Held - Directly Managed | Other government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	600,001
Fair Value	617,710
Amount at which shown in the balance sheet	617,710
Funds Held - Directly Managed | Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	4,398,126
Fair Value	4,615,896
Amount at which shown in the balance sheet	4,615,896
Funds Held - Directly Managed | Municipal
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	141,905
Fair Value	152,010
Amount at which shown in the balance sheet	152,010
Funds Held - Directly Managed | Residential mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	339,228
Fair Value	344,617
Amount at which shown in the balance sheet	344,617
Funds Held - Directly Managed | Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	783,675
Fair Value	801,472
Amount at which shown in the balance sheet	801,472
Funds Held - Directly Managed | Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	562,020
Fair Value	558,217
Amount at which shown in the balance sheet	558,217
Available-for-sale securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	2,036,469
Fair Value	2,035,744
Amount at which shown in the balance sheet	2,035,744
Available-for-sale securities | U.S. government and agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	410,842
Fair Value	410,312
Amount at which shown in the balance sheet	410,312
Available-for-sale securities | U.K. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	15,067
Fair Value	15,349
Amount at which shown in the balance sheet	15,349
Available-for-sale securities | Other government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	84,116
Fair Value	85,147
Amount at which shown in the balance sheet	85,147
Available-for-sale securities | Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	1,081,713
Fair Value	1,081,171
Amount at which shown in the balance sheet	1,081,171
Available-for-sale securities | Municipal
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	15,963
Fair Value	15,872
Amount at which shown in the balance sheet	15,872
Available-for-sale securities | Residential mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	127,704
Fair Value	127,219
Amount at which shown in the balance sheet	127,219
Available-for-sale securities | Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	98,928
Fair Value	98,557
Amount at which shown in the balance sheet	98,557
Available-for-sale securities | Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Cost	202,136
Fair Value	202,117
Amount at which shown in the balance sheet	202,117
Mitchell International | Affiliated entity
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value of other investments in funds or companies owned by or affiliated with related parties	$ 25,000	$ 25,000