Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	$ 4,111,611		$ 4,216,237
Fixed maturities available for sale, at fair value	14,128,989		13,876,003
Equity securities, at fair value	534,482		495,804
Cash	526,628		606,199
Accrued investment income	114,307		113,133
Premiums receivable	1,351,310		1,135,249
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	2,727,303		2,540,143
Ceded unearned premiums	1,014,663		926,611
Deferred acquisition costs net	569,817		535,824
Receivable for securities sold	143,809		205,536
Goodwill and intangible assets	593,008		652,611	$ 712,975
Other assets	1,000,471		1,053,009
Total assets	31,853,120		32,051,658
Reserve for losses and loss adjustment expenses	11,424,337	$ 11,496,205	11,383,792	10,520,511	$ 10,296,821	$ 10,200,960
Unearned premiums	3,833,540		3,622,314
Reinsurance balances payable	411,082		323,496
Revolving credit agreement borrowings	572,289		816,132
Payable for securities purchased	356,583		449,186
Other liabilities	752,399		782,717
Total liabilities of consolidated VIE	21,622,107		21,805,723
Redeemable noncontrolling interests	206,105		205,922
Watford Re
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests	206,105	$ 206,013	205,922	$ 205,736	$ 205,644	$ 205,553
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option	2,385,316		2,426,066
Fixed maturities available for sale, at fair value	279,177		0
Equity securities, at fair value	63,010		0
Cash	45,644		54,503
Accrued investment income	18,108		18,261
Premiums receivable	232,842		177,492
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses	54,299		42,777
Ceded unearned premiums	55,853		24,762
Deferred acquisition costs net	88,752		85,961
Receivable for securities sold	31,387		36,374
Goodwill and intangible assets	7,650		7,650
Other assets	75,478		140,808
Total assets	3,337,516		3,014,654
Reserve for losses and loss adjustment expenses	899,395		798,262
Unearned premiums	384,537		330,644
Reinsurance balances payable	41,185		18,424
Revolving credit agreement borrowings	447,289		441,132
Payable for securities purchased	132,164		42,501
Other liabilities	244,320		215,186
Total liabilities of consolidated VIE	2,148,890		1,846,149
Redeemable noncontrolling interests	$ 220,805		$ 220,622