Premiums Written and Earned - Schedule of Net Premiums Written and Earned (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
Effects of Reinsurance [Line Items]
Gross, premiums written									$ 1,103,393	$ 1,316,097	$ 1,068,453
Ceded, premiums written									(202,434)	(334,197)	(458,063)
Net, premiums written									900,959	981,900	610,390
Gross, premiums earned									1,422,058	1,225,711	1,047,287
Ceded, premiums earned									(267,197)	(330,136)	(434,166)
Net, premiums earned	$ 272,686	$ 270,325	$ 276,563	$ 335,287	$ 231,947	$ 264,597	$ 228,812	$ 170,219	1,154,861	895,575	613,121
Operating Segments | Non-life Run-off
Effects of Reinsurance [Line Items]
Gross, premiums written									(25,069)	(8,910)	14,102
Ceded, premiums written									(269)	(307)	(7,620)
Net, premiums written									(25,338)	(9,217)	6,482
Gross, premiums earned									197,009	25,230	23,950
Ceded, premiums earned									(28,513)	(15,803)	(9,788)
Net, premiums earned									168,496	9,427	14,162
Operating Segments | Atrium
Effects of Reinsurance [Line Items]
Gross, premiums written									192,373	171,494	153,472
Ceded, premiums written									(20,017)	(18,006)	(19,258)
Net, premiums written									172,356	153,488	134,214
Gross, premiums earned									182,678	164,428	152,278
Ceded, premiums earned									(18,619)	(18,113)	(17,531)
Net, premiums earned									164,059	146,315	134,747
Operating Segments | StarStone
Effects of Reinsurance [Line Items]
Gross, premiums written									917,555	1,121,135	895,160
Ceded, premiums written									(182,126)	(315,573)	(430,259)
Net, premiums written									735,429	805,562	464,901
Gross, premiums earned									1,021,827	1,010,816	865,159
Ceded, premiums earned									(219,901)	(295,857)	(405,756)
Net, premiums earned									801,926	714,959	459,403
Other
Effects of Reinsurance [Line Items]
Gross, premiums written									18,534	32,378	5,719
Ceded, premiums written									(22)	(311)	(926)
Net, premiums written									18,512	32,067	4,793
Gross, premiums earned									20,544	25,237	5,900
Ceded, premiums earned									(164)	(363)	(1,091)
Net, premiums earned									$ 20,380	$ 24,874	$ 4,809