Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		$ 3,520,771		$ 3,663,477
Fixed maturities available for sale, at fair value		17,207,731		16,894,526
Equity securities, at fair value		1,257,317		838,925
Cash		854,259		726,230
Accrued investment income		102,064		117,937
Premiums receivable		2,203,753	$ 2,155,204	1,778,717
Reinsurance recoverable on unpaid and paid losses and LAE		4,363,507	4,303,135	4,346,816
Ceded unearned premiums		1,364,603		1,234,683
Deferred acquisition costs		714,531		633,400
Receivable for securities sold		167,281		24,133
Goodwill and intangible assets		688,490		738,083	$ 641,010
Other assets		1,632,756		1,383,788
Total assets		41,199,648		37,885,361
Reserve for losses and loss adjustment expenses		15,044,874	14,309,580	13,891,842	12,230,316	$ 12,010,041	$ 11,853,297
Unearned premiums		4,827,445		4,339,549
Reinsurance balances payable		793,467		667,072
Revolving credit agreement borrowings		335,587		484,287
Senior notes		2,860,733		1,871,626
Payable for securities purchased		275,257		87,579
Other liabilities		1,467,739		1,513,330
Total liabilities		28,472,748		25,569,809
Redeemable noncontrolling interests		55,986		55,404
Watford Holdings Ltd
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests		55,986	$ 55,376	55,404	$ 206,475	$ 206,383	$ 206,292
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		1,886,676		1,898,091
Fixed maturities available for sale, at fair value		690,225		745,708
Equity securities, at fair value		62,443		65,338
Cash		107,653		102,437
Accrued investment income		14,364		14,025
Premiums receivable		258,178		273,657
Reinsurance recoverable on unpaid and paid losses and LAE		229,746		170,973
Ceded unearned premiums		131,919		132,577
Deferred acquisition costs		64,149		64,044
Receivable for securities sold		31,314		16,287
Goodwill and intangible assets		7,650		7,650
Other assets		63,441		60,070
Total assets		3,547,758		3,550,857
Reserve for losses and loss adjustment expenses		1,353,049		1,263,628
Unearned premiums		456,170		438,907
Reinsurance balances payable		72,776		77,066
Revolving credit agreement borrowings		335,587		484,287
Senior notes		172,554		172,418
Payable for securities purchased		67,272		18,180
Other liabilities	[1]	261,267		171,714
Total liabilities		2,718,675		2,626,200
Redeemable noncontrolling interests		$ 52,351		$ 52,305

[1] Includes certain borrowings related to investing activities.