Losses and Loss Adjustment Expenses - Reconciliation of Beginning and Ending Reserves for Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				$ 9,409,504				$ 7,398,088	$ 9,409,504	$ 7,398,088	$ 5,987,867
Less: reinsurance reserves recoverable				1,867,841				1,870,033	1,867,841	1,870,033	1,388,193
Less: deferred charges on retroactive reinsurance				86,585				80,192	86,585	80,192	94,551
Net balance as at January 1				7,455,078				5,447,863	7,455,078	5,447,863	4,505,123
Net incurred losses and LAE:
Current period									823,658	689,782	437,853
Prior periods									48,917	(235,757)	(244,302)
Total net incurred losses and LAE	$ 121,416	$ 222,417	$ 216,338	312,404	$ 187,698	$ 153,974	$ 92,819	19,534	872,575	454,025	193,551
Net paid losses:
Current period									(200,264)	(189,560)	(82,273)
Prior periods									(1,669,004)	(1,194,985)	(862,921)
Total net paid losses									(1,869,268)	(1,384,545)	(945,194)
Effect of exchange rate movement									48,695	(145,243)	158,429
Acquired on purchase of subsidiaries									686	1,310,874	10,251
Assumed business									1,586,307	1,772,104	1,525,703
Ceded business									(47,018)	0	0
Net balance as at December 31	8,047,055				7,455,078				8,047,055	7,455,078	5,447,863
Plus: reinsurance reserves recoverable	2,109,721				1,867,841				2,109,721	1,867,841	1,870,033
Plus: deferred charges on retroactive reinsurance	272,462				86,585				272,462	86,585	80,192
Balance as at December 31	10,429,238				9,409,504				10,429,238	9,409,504	7,398,088
Non-life Run-off
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				7,540,662				5,949,472	7,540,662	5,949,472	4,716,363
Less: reinsurance reserves recoverable				1,366,123				1,377,485	1,366,123	1,377,485	1,000,953
Less: deferred charges on retroactive reinsurance				86,585				80,192	86,585	80,192	94,551
Net balance as at January 1				6,087,954				4,491,795	6,087,954	4,491,795	3,620,859
Net incurred losses and LAE:
Current period									123,559	12,451	5,866
Prior periods									(71,934)	(318,518)	(196,540)
Total net incurred losses and LAE									51,625	(306,067)	(190,674)
Net paid losses:
Current period									(64,820)	(5)	(2,835)
Prior periods									(1,182,804)	(838,812)	(578,888)
Total net paid losses									(1,247,624)	(838,817)	(581,723)
Effect of exchange rate movement									47,355	(132,632)	138,772
Acquired on purchase of subsidiaries									686	1,111,839	10,251
Assumed business									1,586,307	1,761,836	1,494,310
Ceded business									(47,018)	0	0
Net balance as at December 31	6,479,285				6,087,954				6,479,285	6,087,954	4,491,795
Plus: reinsurance reserves recoverable	1,543,614				1,366,123				1,543,614	1,366,123	1,377,485
Plus: deferred charges on retroactive reinsurance	272,462				86,585				272,462	86,585	80,192
Balance as at December 31	8,295,361				7,540,662				8,295,361	7,540,662	5,949,472
Atrium
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				241,284				240,873	241,284	240,873	212,122
Less: reinsurance reserves recoverable				38,768				40,531	38,768	40,531	30,009
Net balance as at January 1				202,516				200,342	202,516	200,342	182,113
Net incurred losses and LAE:
Current period									85,027	83,627	90,359
Prior periods									(7,751)	(13,817)	(20,940)
Total net incurred losses and LAE									77,276	69,810	69,419
Net paid losses:
Current period									(34,617)	(35,537)	(24,571)
Prior periods									(43,572)	(28,969)	(31,107)
Total net paid losses									(78,189)	(64,506)	(55,678)
Effect of exchange rate movement									1,253	(3,130)	4,488
Net balance as at December 31	202,856				202,516				202,856	202,516	200,342
Plus: reinsurance reserves recoverable	28,816				38,768				28,816	38,768	40,531
Balance as at December 31	231,672				241,284				231,672	241,284	240,873
StarStone
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance as at January 1				1,608,697				1,207,743	1,608,697	1,207,743	1,059,382
Less: reinsurance reserves recoverable				462,950				452,017	462,950	452,017	357,231
Net balance as at January 1				$ 1,145,747				$ 755,726	1,145,747	755,726	702,151
Net incurred losses and LAE:
Current period									598,468	578,892	341,628
Prior periods									129,168	94,491	(26,822)
Total net incurred losses and LAE									727,636	673,383	314,806
Net paid losses:
Current period									(96,261)	(150,778)	(54,867)
Prior periods									(435,372)	(326,352)	(252,926)
Total net paid losses									(531,633)	(477,130)	(307,793)
Effect of exchange rate movement									87	(9,481)	15,169
Acquired on purchase of subsidiaries									0	192,981	0
Assumed business									0	10,268	31,393
Ceded business									0	0	0
Net balance as at December 31	1,341,837				1,145,747				1,341,837	1,145,747	755,726
Plus: reinsurance reserves recoverable	537,291				462,950				537,291	462,950	452,017
Balance as at December 31	$ 1,879,128				$ 1,608,697				$ 1,879,128	$ 1,608,697	$ 1,207,743