Variable Interest Entities and Noncontrolling Interests - Carrying amount of assets and liabilities of variable interest entity (Details) - USD ($) $ in Thousands		Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		$ 3,838,243		$ 3,983,571
Fixed maturities available for sale, at fair value		16,470,523		14,699,010
Equity securities, at fair value		550,485		338,899
Cash		880,099		646,556
Accrued investment income		116,196		114,641
Premiums receivable		1,618,186		1,299,150
Reinsurance recoverable on unpaid and paid losses and LAE		3,168,195		2,919,372
Ceded unearned premiums		1,168,258		975,469
Deferred acquisition costs		622,028		569,574
Receivable for securities sold		50,615		36,246
Goodwill and intangible assets		624,500		634,920	$ 566,662
Other assets		1,192,093		929,611
Total assets		35,572,603		32,218,329
Reserve for losses and loss adjustment expenses		12,389,384	$ 12,230,316	11,853,297	11,554,321	$ 11,424,337	$ 11,383,792
Unearned premiums		4,243,372		3,753,636
Reinsurance balances payable		601,891		393,107
Revolving credit agreement borrowings		490,720		455,682
Senior notes		1,871,386		1,733,528
Payable for securities purchased		176,130		90,034
Other liabilities		1,007,524		911,500
Total liabilities of consolidated VIE		23,510,794		21,780,650
Redeemable noncontrolling interests		48,789		206,292
Watford Holdings Ltd
Variable Interest Entity [Line Items]
Redeemable noncontrolling interests		48,789	$ 206,475	206,292	$ 206,199	$ 206,105	$ 205,922
Variable Interest Entity, Primary Beneficiary | Watford Holdings Ltd
Variable Interest Entity [Line Items]
Investments accounted for using the fair value option		2,048,295		2,312,003
Fixed maturities available for sale, at fair value		678,094		393,351
Equity securities, at fair value		43,488		32,206
Cash		80,390		63,529
Accrued investment income		18,277		19,461
Premiums receivable		302,265		227,301
Reinsurance recoverable on unpaid and paid losses and LAE		144,437		86,445
Ceded unearned premiums		129,909		61,587
Deferred acquisition costs		67,241		80,858
Receivable for securities sold		25,283		24,507
Goodwill and intangible assets		7,650		7,650
Other assets		66,165		63,959
Total assets		3,611,494		3,372,857
Reserve for losses and loss adjustment expenses		1,164,945		1,032,760
Unearned premiums		454,148		390,114
Reinsurance balances payable		79,264		21,034
Revolving credit agreement borrowings		490,720		455,682
Senior notes		172,350		0
Payable for securities purchased		40,586		60,142
Other liabilities	[1]	196,431		302,524
Total liabilities of consolidated VIE		2,598,444		2,262,256
Redeemable noncontrolling interests		$ 52,281		$ 220,992

[1] Includes certain borrowings related to investing activities.