Losses and Loss Adjustment Expenses - Additional Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) commutation	Dec. 31, 2018 USD ($) commutation policy_buy_back	Dec. 31, 2017 USD ($)
Effects of Reinsurance [Line Items]
Net liabilities relating to direct asbestos and environmental exposures	$ 2,078,300				$ 1,703,600				$ 2,078,300	$ 1,703,600
Asbestos and environmental liabilities	2,259,600				1,839,700				2,259,600	1,839,700
Increase (decrease) in asbestos and environmental reserves, gross									419,900	(152,400)
Increase (reduction) in estimated future expenses									374,700	(159,600)
Net incurred losses and loss adjustment expenses	121,416	$ 222,417	$ 216,338	$ 312,404	$ 187,698	$ 153,974	$ 92,819	$ 19,534	872,575	454,025	$ 193,551
Net losses paid, prior period									1,669,004	1,194,985	862,921
Net incurred losses and LAE, current period									823,658	689,782	437,853
Net incurred losses and LAE, prior period									$ 48,917	(235,757)	(244,302)
Atrium Segment
Effects of Reinsurance [Line Items]
Syndicate underwriting capacity percentage									25.00%
Net incurred losses and loss adjustment expenses									$ 77,276	69,810	69,419
Increase (reduction) in estimates of net ultimate losses, prior period									(8,086)	(8,699)	(17,867)
Downward incurred loss development									(38,380)	(27,507)	(35,650)
Net change in case and LAE reserves, prior period									(13,278)	(10,161)	(13,324)
Net losses paid, prior period									43,572	28,969	31,107
Net incurred losses and LAE, current period									85,027	83,627	90,359
Net incurred losses and LAE, prior period									(7,751)	(13,817)	(20,940)
Amortization of fair value adjustments, prior period									335	(5,118)	(2,720)
Reduction in provisions for bad debt, prior period									0	0	89
Increase (reduction) in provisions for unallocated LAE, prior period									0	0	(442)
Non-life Run-off
Effects of Reinsurance [Line Items]
Net incurred losses and loss adjustment expenses									51,625	(306,067)	(190,674)
Increase (reduction) in estimates of net ultimate losses, prior period									(219,968)	(286,439)	(195,662)
Downward incurred loss development									(848,776)	(573,127)	(393,100)
Net change in case and LAE reserves, prior period									(553,996)	(552,124)	(381,450)
Net losses paid, prior period									1,182,804	838,812	578,888
Net incurred losses and LAE, current period									123,559	12,451	5,866
Net incurred losses and LAE, prior period									(71,934)	(318,518)	(196,540)
Increase (reduction) in provisions for unallocated LAE									(57,800)	(65,400)
Amortization of deferred charges, prior period									38,627	13,781	14,359
Amortization of fair value adjustments, prior period									50,070	12,877	10,114
Changes in fair value, fair value option									117,181	6,664	30,256
Net reduction in case and IBNR reserves											(774,600)
Reduction in provisions for bad debt, prior period									0	0	(1,536)
Increase (reduction) in provisions for unallocated LAE, prior period									(57,844)	(65,401)	(54,071)
Non-life Run-off | Workers' compensation
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									(136,418)	(154,560)	(155,009)
Downward incurred loss development									(188,944)	(115,648)	(151,797)
Net change in case and LAE reserves, prior period									(156,435)	(178,138)	(190,924)
Net losses paid, prior period									208,961	139,226	187,712
Net reduction in case and IBNR reserves									(1,402,800)	$ (1,125,300)
Reduction of open inventory claims, percentage										78.00%
Non-life Run-off | Workers' compensation, two portfolios
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									39,300
Downward incurred loss development									14,300
Net change in case and LAE reserves, prior period									53,600
Non-life Run-off | Six Communications
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									$ (6,100)
Number of commutations | commutation									6
Non-life Run-off | Seven Communications
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period										$ (11,200)
Number of commutations | commutation										7
Non-life Run-off | Professional indemnity/Directors & Officers
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									$ (38,557)	$ 19,681	(10,211)
Downward incurred loss development									(104,984)	(130,957)	(24,559)
Net change in case and LAE reserves, prior period									(36,986)	(11,159)	(19,054)
Net losses paid, prior period									103,413	161,797	33,402
Ceded reinsurance asset increase	$ 13,500								13,500
Non-life Run-off | Asbestos
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									6,811	(64,949)	27,029
Downward incurred loss development									(146,749)	(151,662)	(76,837)
Net change in case and LAE reserves, prior period									35,003	(21,535)	(1,865)
Net losses paid, prior period									118,557	$ 108,248	105,731
Volume of claims reported, percentage, less than										3.00%
Average cost per claims, percentage										5.00%
Additional reserves recorded											60,500
Favorable development in other portfolios											(33,500)
Non-life Run-off | Asbestos Exposure, Six Commutations
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									$ (9,800)
Number of commutations | commutation									6
Non-life Run-off | Asbestos Exposure, Eight Communications and Two Policy Buy-Backs
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period										$ (9,500)
Number of commutations | commutation										8
Number of policy buy-backs | policy_buy_back										2
Non-life Run-off | Construction defect
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period									$ (1,260)	$ (18,221)	(33,025)
Downward incurred loss development									(25,025)	(33,146)	(42,804)
Net change in case and LAE reserves, prior period									(8,313)	(7,257)	(24,023)
Net losses paid, prior period									$ 32,078	22,182	$ 33,802
Reduction of open inventory claims, percentage											73.00%
Segment Concentration Risk | Liability Claims and Claims Adjustment Expense | Atrium Segment
Effects of Reinsurance [Line Items]
Percent of total investments									2.00%
Short-duration Insurance Contracts, Accident Years 1995-2005 | Non-life Run-off | Workers' compensation
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period										22,700
Downward incurred loss development										14,300
Net change in case and LAE reserves, prior period										37,000
Short-duration Insurance Contracts, Accident Years 2010-2014 | Non-life Run-off | Workers' compensation
Effects of Reinsurance [Line Items]
Increase (reduction) in estimates of net ultimate losses, prior period										$ 30,200