Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	$ 3,827,408		$ 3,421,220
Cash	740,320		842,942	$ 516,591		$ 553,326
Accrued investment income	108,562		124,483
Premiums receivable	1,314,564		1,072,435
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,155,107		2,114,138
Ceded unearned premiums	961,330		859,567
Deferred acquisition costs	506,748		447,560
Receivable for securities sold	212,512		58,284
Goodwill and intangible assets	712,975		781,553	88,327
Other assets	1,014,282		889,080
Total assets of consolidated VIE	30,862,639		29,372,109
Reserve for losses and loss adjustment expenses	10,520,511	$ 10,296,821	10,200,960	9,471,647	$ 9,378,987	9,125,250
Unearned premiums	3,679,651		3,406,870
Reinsurance balances payable	361,000		300,407
Revolving credit agreement borrowings	686,452		756,650
Payable for securities purchased	458,722		76,183
Other liabilities	648,099		806,260
Total liabilities of consolidated VIE	20,880,560		20,060,984
Redeemable noncontrolling interests	205,736		205,553
Watford Re
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests	205,736	$ 205,644	205,553	$ 205,366	$ 205,274	$ 205,182
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	2,129,436		1,857,623
Cash	63,929		74,893
Accrued investment income	13,229		17,017
Premiums receivable	193,769		189,911
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	28,267		24,420
Ceded unearned premiums	20,455		12,145
Deferred acquisition costs	91,183		86,379
Receivable for securities sold	32,618		1,326
Goodwill and intangible assets	7,650		7,650
Other assets	132,068		111,386
Total assets of consolidated VIE	2,712,604		2,382,750
Reserve for losses and loss adjustment expenses	643,424		510,809
Unearned premiums	314,446		293,480
Reinsurance balances payable	17,623		12,289
Revolving credit agreement borrowings	186,452		256,650
Payable for securities purchased	224,152		42,922
Other liabilities	119,928		88,976
Total liabilities of consolidated VIE	1,506,025		1,205,126
Redeemable noncontrolling interests	$ 220,436		$ 220,253