Losses and Loss Adjustment Expenses - Net Incurred Losses by Line of Business (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	$ 1,669,004	$ 1,194,985	$ 862,921
Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	1,182,804	838,812	578,888
Net change in case and LAE reserves, prior period	(553,996)	(552,124)	(381,450)
Net change in IBNR reserves, prior period	(848,776)	(573,127)	(393,100)
Increase (reduction) in estimates of net ultimate losses, prior period	(219,968)	(286,439)	(195,662)
Asbestos | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	118,557	108,248	105,731
Net change in case and LAE reserves, prior period	35,003	(21,535)	(1,865)
Net change in IBNR reserves, prior period	(146,749)	(151,662)	(76,837)
Increase (reduction) in estimates of net ultimate losses, prior period	6,811	(64,949)	27,029
Environmental | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	16,899	21,273	26,542
Net change in case and LAE reserves, prior period	13,796	479	(9,438)
Net change in IBNR reserves, prior period	(15,707)	(7,599)	(7,748)
Increase (reduction) in estimates of net ultimate losses, prior period	14,988	14,153	9,356
General casualty | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	175,044	141,624	94,526
Net change in case and LAE reserves, prior period	(89,968)	(115,240)	(54,292)
Net change in IBNR reserves, prior period	(91,818)	(60,828)	(49,025)
Increase (reduction) in estimates of net ultimate losses, prior period	(6,742)	(34,444)	(8,791)
Workers' compensation/personal accident | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	208,961	139,226	187,712
Net change in case and LAE reserves, prior period	(156,435)	(178,138)	(190,924)
Net change in IBNR reserves, prior period	(188,944)	(115,648)	(151,797)
Increase (reduction) in estimates of net ultimate losses, prior period	(136,418)	(154,560)	(155,009)
Marine, aviation and transit | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	82,058	67,831	18,272
Net change in case and LAE reserves, prior period	(77,958)	(44,200)	(9,322)
Net change in IBNR reserves, prior period	(24,508)	(21,188)	(11,517)
Increase (reduction) in estimates of net ultimate losses, prior period	(20,408)	2,443	(2,567)
Construction defect | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	32,078	22,182	33,802
Net change in case and LAE reserves, prior period	(8,313)	(7,257)	(24,023)
Net change in IBNR reserves, prior period	(25,025)	(33,146)	(42,804)
Increase (reduction) in estimates of net ultimate losses, prior period	(1,260)	(18,221)	(33,025)
Professional indemnity/Directors & Officers | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	103,413	161,797	33,402
Net change in case and LAE reserves, prior period	(36,986)	(11,159)	(19,054)
Net change in IBNR reserves, prior period	(104,984)	(130,957)	(24,559)
Increase (reduction) in estimates of net ultimate losses, prior period	(38,557)	19,681	(10,211)
Motor | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	276,563	104,182	24,391
Net change in case and LAE reserves, prior period	(134,127)	(109,962)	(15,990)
Net change in IBNR reserves, prior period	(179,887)	(34,215)	(8,513)
Increase (reduction) in estimates of net ultimate losses, prior period	(37,451)	(39,995)	(112)
Property | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	94,093	22,178	13,440
Net change in case and LAE reserves, prior period	(73,259)	(24,271)	(11,196)
Net change in IBNR reserves, prior period	(7,358)	(11,497)	(5,162)
Increase (reduction) in estimates of net ultimate losses, prior period	13,476	(13,590)	(2,918)
Other | Non-life Run-off
Liability for Claims and Claims Adjustment Expense [Line Items]
Net losses paid, prior period	75,138	50,271	41,070
Net change in case and LAE reserves, prior period	(25,749)	(40,841)	(45,346)
Net change in IBNR reserves, prior period	(63,796)	(6,387)	(15,138)
Increase (reduction) in estimates of net ultimate losses, prior period	$ (14,407)	$ 3,043	$ (19,414)