Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		$ 3,310,517	$ 3,663,477
Fixed maturities available for sale, at fair value		16,841,571	16,894,526
Equity securities, at fair value		1,181,903	838,925
Cash		882,284	726,230
Accrued investment income		118,089	117,937
Premiums receivable (net of allowance for credit losses: $27,990 and $21,003)		2,155,204	1,778,717
Reinsurance recoverable on unpaid and paid losses and LAE		4,303,135	4,346,816
Ceded unearned premiums		1,357,284	1,234,683
Deferred acquisition costs		708,848	633,400
Receivable for securities sold		221,573	24,133
Goodwill and intangible assets		705,450	738,083	$ 659,215
Other assets		1,509,232	1,383,788
Total assets		38,416,126	37,885,361
Reserve for losses and loss adjustment expenses		14,309,580	13,891,842	12,010,041	$ 11,853,297
Unearned premiums		4,817,191	4,339,549
Reinsurance balances payable		737,597	667,072
Revolving credit agreement borrowings		500,587	484,287
Senior notes		1,871,869	1,871,626
Payable for securities purchased		327,359	87,579
Other liabilities		1,392,905	1,513,330
Total liabilities of consolidated VIE		26,500,721	25,569,809
Redeemable noncontrolling interests		55,376	55,404
Watford Holdings Ltd
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests		55,376	55,404	$ 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		1,732,086	1,898,091
Fixed maturities available for sale, at fair value		717,552	745,708
Equity securities, at fair value		63,169	65,338
Cash		96,580	102,437
Accrued investment income		16,344	14,025
Premiums receivable (net of allowance for credit losses: $27,990 and $21,003)		281,541	273,657
Reinsurance recoverable on unpaid and paid losses and LAE		197,458	170,973
Ceded unearned premiums		128,570	132,577
Deferred acquisition costs		71,402	64,044
Receivable for securities sold		26,789	16,287
Goodwill and intangible assets		7,650	7,650
Other assets		67,493	60,070
Total assets		3,406,634	3,550,857
Reserve for losses and loss adjustment expenses		1,300,249	1,263,628
Unearned premiums		478,663	438,907
Reinsurance balances payable		71,204	77,066
Revolving credit agreement borrowings		500,587	484,287
Senior notes		172,486	172,418
Payable for securities purchased		63,829	18,180
Other liabilities	[1]	203,234	171,714
Total liabilities of consolidated VIE		2,790,252	2,626,200
Redeemable noncontrolling interests		$ 52,328	$ 52,305

[1] Includes certain borrowings related to investing activities.