Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		$ 3,749,575		$ 3,663,477
Fixed maturities available for sale, at fair value		18,452,888		16,894,526
Equity securities, at fair value		1,502,015		838,925
Cash		976,398		726,230
Accrued investment income		106,940		117,937
Premiums receivable		2,225,311	$ 2,203,753	1,778,717
Reinsurance recoverable on unpaid and paid losses and LAE		4,621,937	4,363,507	4,346,816
Ceded unearned premiums		1,450,200		1,234,683
Deferred acquisition costs		750,901		633,400
Receivable for securities sold		158,674		24,133
Goodwill and intangible assets		713,777		738,083	$ 624,500
Other assets		1,771,998		1,383,788
Total assets		42,716,035		37,885,361
Reserve for losses and loss adjustment expenses		15,900,526	15,044,874	13,891,842	12,389,384	$ 12,230,316	$ 11,853,297
Unearned premiums		5,062,052		4,339,549
Reinsurance balances payable		873,067		667,072
Revolving credit agreement borrowings		210,687		484,287
Senior notes		2,860,811		1,871,626
Payable for securities purchased		382,236		87,579
Other liabilities		1,681,181		1,513,330
Total liabilities		29,448,283		25,569,809
Redeemable noncontrolling interests		57,835		55,404
Watford Holdings Ltd
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests		57,835	$ 55,986	55,404	$ 48,789	$ 206,475	$ 206,292
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		1,937,037		1,898,091
Fixed maturities available for sale, at fair value		649,781		745,708
Equity securities, at fair value		52,807		65,338
Cash		195,333		102,437
Accrued investment income		16,234		14,025
Premiums receivable		263,748		273,657
Reinsurance recoverable on unpaid and paid losses and LAE		263,293		170,973
Ceded unearned premiums		131,885		132,577
Deferred acquisition costs		58,583		64,044
Receivable for securities sold		1,730		16,287
Goodwill and intangible assets		7,650		7,650
Other assets		75,139		60,070
Total assets		3,653,220		3,550,857
Reserve for losses and loss adjustment expenses		1,429,656		1,263,628
Unearned premiums		459,476		438,907
Reinsurance balances payable		68,339		77,066
Revolving credit agreement borrowings		210,687		484,287
Senior notes		172,621		172,418
Payable for securities purchased		76,567		18,180
Other liabilities	[1]	316,599		171,714
Total liabilities		2,733,945		2,626,200
Redeemable noncontrolling interests		$ 52,375		$ 52,305

[1] Includes certain borrowings related to investing activities.