Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		$ 3,721,035		$ 3,983,571
Fixed maturities available for sale, at fair value		15,881,732		14,699,010
Equity securities, at fair value		670,943		338,899
Cash		605,316		646,556
Accrued investment income		119,252		114,641
Premiums receivable		1,606,040		1,299,150
Reinsurance recoverable on unpaid and paid losses and LAE		3,171,257		2,919,372
Ceded unearned premiums		1,136,728		975,469
Deferred acquisition costs		600,740		569,574
Receivable for securities sold		164,592		36,246
Goodwill and intangible assets		641,010		634,920	$ 593,008
Other assets		1,135,718		929,611
Total assets		34,851,670		32,218,329
Reserve for losses and loss adjustment expenses		12,230,316	$ 12,010,041	11,853,297	11,424,337	$ 11,496,205	$ 11,383,792
Unearned premiums		4,056,860		3,753,636
Reinsurance balances payable		531,990		393,107
Revolving credit agreement borrowings		491,006		455,682
Payable for securities purchased		294,109		90,034
Other liabilities		904,438		911,500
Total liabilities of consolidated VIE		23,032,496		21,780,650
Redeemable noncontrolling interests		206,475		206,292
Watford Re
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests		206,475	$ 206,383	206,292	$ 206,105	$ 206,013	$ 205,922
Variable Interest Entity, Primary Beneficiary | Watford Re
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		1,972,947		2,312,003
Fixed maturities available for sale, at fair value		732,454		393,351
Equity securities, at fair value		64,703		32,206
Cash		68,977		63,529
Accrued investment income		16,916		19,461
Premiums receivable		228,588		227,301
Reinsurance recoverable on unpaid and paid losses and LAE		123,961		86,445
Ceded unearned premiums		79,513		61,587
Deferred acquisition costs		71,557		80,858
Receivable for securities sold		29,425		24,507
Goodwill and intangible assets		7,650		7,650
Other assets		71,837		63,959
Total assets		3,468,528		3,372,857
Reserve for losses and loss adjustment expenses		1,126,080		1,032,760
Unearned premiums		375,323		390,114
Reinsurance balances payable		23,312		21,034
Revolving credit agreement borrowings		491,006		455,682
Payable for securities purchased		51,216		60,142
Other liabilities	[1]	219,120		302,524
Total liabilities of consolidated VIE		2,286,057		2,262,256
Redeemable noncontrolling interests		$ 221,175		$ 220,992

[1] Includes certain borrowings related to investing activities.