Reinsurance Balances Recoverable on Paid and Unpaid Losses - Summary of Reinsurance Reserves Recoverable (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Outstanding losses	$ 1,309,313	$ 1,183,728
IBNR	903,749	830,465
Fair value adjustments - acquired companies	(15,255)	(15,613)
Fair value adjustments - fair value option	(88,086)	(130,739)
Total reinsurance reserves recoverable	2,109,721	1,867,841	$ 1,870,033	$ 1,388,193
Paid losses recoverable	270,169	161,822
Net	2,379,890	2,029,663
Reinsurance balances recoverable on paid and unpaid losses (Note 8)	1,684,372	1,290,072
Reinsurance balances recoverable on paid and unpaid losses - fair value option	695,518	739,591
Non-life Run-off
Effects of Reinsurance [Line Items]
Outstanding losses	972,293	901,772
IBNR	673,059	609,434
Fair value adjustments - acquired companies	(13,652)	(14,344)
Fair value adjustments - fair value option	(88,086)	(130,739)
Total reinsurance reserves recoverable	1,543,614	1,366,123	1,377,485	1,000,953
Paid losses recoverable	181,375	138,265
Net	1,724,989	1,504,388
Reinsurance balances recoverable on paid and unpaid losses (Note 8)	1,029,471	764,797
Reinsurance balances recoverable on paid and unpaid losses - fair value option	695,518	739,591
Atrium
Effects of Reinsurance [Line Items]
Outstanding losses	9,011	18,891
IBNR	19,286	19,247
Fair value adjustments - acquired companies	519	630
Fair value adjustments - fair value option	0	0
Total reinsurance reserves recoverable	28,816	38,768	40,531	30,009
Paid losses recoverable	1,541	(256)
Net	30,357	38,512
Reinsurance balances recoverable on paid and unpaid losses (Note 8)	30,357	38,512
Reinsurance balances recoverable on paid and unpaid losses - fair value option	0	0
StarStone
Effects of Reinsurance [Line Items]
Outstanding losses	328,009	263,065
IBNR	211,404	201,784
Fair value adjustments - acquired companies	(2,122)	(1,899)
Fair value adjustments - fair value option	0	0
Total reinsurance reserves recoverable	537,291	462,950	$ 452,017	$ 357,231
Paid losses recoverable	87,253	23,813
Net	624,544	486,763
Reinsurance balances recoverable on paid and unpaid losses (Note 8)	624,544	486,763
Reinsurance balances recoverable on paid and unpaid losses - fair value option	$ 0	$ 0