Losses and Loss Adjustment Expenses - Net Incurred Losses and Loss Adjustment Expense Liabilities (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
Prior Period
Net losses paid, prior period									$ 1,669,004	$ 1,194,985	$ 862,921
Prior periods									48,917	(235,757)	(244,302)
Current Period
Net losses paid, current period									200,264	189,560	82,273
Net incurred losses and LAE, current period									823,658	689,782	437,853
Total
Net losses paid									1,869,268	1,384,545	945,194
Net change in case and LAE reserves									(497,930)	(460,394)	(341,030)
Net change in IBNR reserves									(651,181)	(439,416)	(406,588)
Increase (reduction) in estimates of net ultimate losses									720,157	484,735	197,576
Increase (reduction) in provisions for bad debt											(1,377)
Increase (reduction) in provisions for unallocated LAE									(53,963)	(58,648)	(53,712)
Amortization of deferred charges									38,627	13,781	14,359
Amortization of fair value adjustments									50,573	7,493	6,449
Total changes in fair value - fair value option									117,181	6,664	30,256
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
Non-life Run-off
Prior Period
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)
Increase (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)
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, prior period									117,181	6,664	30,256
Prior periods									(71,934)	(318,518)	(196,540)
Current Period
Net losses paid, current period									64,820	5	2,835
Net change in case and LAE reserves, current period									23,105	4,704	397
Net change in IBNR reserves, current period									35,194	7,742	2,373
Increase (reduction) in estimates of net ultimate losses, current period									123,119	12,451	5,605
Increase (reduction) in provisions for bad debt, current period									0	0	0
Increase (reduction) in provisions for unallocated LAE, current period									440	0	261
Amortization of deferred charges, current period									0	0	0
Amortization of fair value adjustments, current period									0	0	0
Changes in fair value - fair value option, current period									0	0	0
Net incurred losses and LAE, current period									123,559	12,451	5,866
Total
Net losses paid									1,247,624	838,817	581,723
Net change in case and LAE reserves									(530,891)	(547,420)	(381,053)
Net change in IBNR reserves									(813,582)	(565,385)	(390,727)
Increase (reduction) in estimates of net ultimate losses									(96,849)	(273,988)	(190,057)
Increase (reduction) in provisions for bad debt									0	0	(1,536)
Increase (reduction) in provisions for unallocated LAE									(57,404)	(65,401)	(53,810)
Amortization of deferred charges									38,627	13,781	14,359
Amortization of fair value adjustments									50,070	12,877	10,114
Total changes in fair value - fair value option									117,181	6,664	30,256
Total net incurred losses and LAE									51,625	(306,067)	(190,674)
Atrium
Prior Period
Net losses paid, prior period									43,572	28,969	31,107
Net change in case and LAE reserves, prior period									(13,278)	(10,161)	(13,324)
Net change in IBNR reserves, prior period									(38,380)	(27,507)	(35,650)
Increase (reduction) in estimates of net ultimate losses, prior period									(8,086)	(8,699)	(17,867)
Increase (reduction) in provisions for bad debt, prior period									0	0	89
Increase (reduction) in provisions for unallocated LAE, prior period									0	0	(442)
Amortization of fair value adjustments, prior period									335	(5,118)	(2,720)
Prior periods									(7,751)	(13,817)	(20,940)
Current Period
Net losses paid, current period									34,617	35,537	24,571
Net change in case and LAE reserves, current period									16,812	16,492	21,662
Net change in IBNR reserves, current period									33,598	31,598	43,329
Increase (reduction) in estimates of net ultimate losses, current period									85,027	83,627	89,562
Increase (reduction) in provisions for bad debt, current period									0	0	70
Increase (reduction) in provisions for unallocated LAE, current period									0	0	727
Amortization of fair value adjustments, current period									0	0	0
Net incurred losses and LAE, current period									85,027	83,627	90,359
Total
Net losses paid									78,189	64,506	55,678
Net change in case and LAE reserves									3,534	6,331	8,338
Net change in IBNR reserves									(4,782)	4,091	7,679
Increase (reduction) in estimates of net ultimate losses									76,941	74,928	71,695
Increase (reduction) in provisions for bad debt									0	0	159
Increase (reduction) in provisions for unallocated LAE									0	0	285
Amortization of deferred charges									0	0	0
Amortization of fair value adjustments									335	(5,118)	(2,720)
Total changes in fair value - fair value option									0	0	0
Total net incurred losses and LAE									77,276	69,810	69,419
StarStone
Prior Period
Net losses paid, prior period									435,372	326,352	252,926
Net change in case and LAE reserves, prior period									(85,983)	(81,491)	(63,785)
Net change in IBNR reserves, prior period									(214,787)	(144,212)	(208,244)
Increase (reduction) in estimates of net ultimate losses, prior period									134,602	100,649	(19,103)
Increase (reduction) in provisions for unallocated LAE, prior period									(5,602)	(5,892)	(6,774)
Amortization of fair value adjustments, prior period									168	(266)	(945)
Prior periods									129,168	94,491	(26,822)
Current Period
Net losses paid, current period									96,261	150,778	54,867
Net change in case and LAE reserves, current period									111,950	157,378	95,470
Net change in IBNR reserves, current period									381,214	258,091	184,704
Increase (reduction) in estimates of net ultimate losses, current period									589,425	566,247	335,041
Increase (reduction) in provisions for unallocated LAE, current period									9,043	12,645	6,587
Amortization of fair value adjustments, current period									0	0	0
Net incurred losses and LAE, current period									598,468	578,892	341,628
Total
Net losses paid									531,633	477,130	307,793
Net change in case and LAE reserves									25,967	75,887	31,685
Net change in IBNR reserves									166,427	113,879	(23,540)
Increase (reduction) in estimates of net ultimate losses									724,027	666,896	315,938
Increase (reduction) in provisions for bad debt											0
Increase (reduction) in provisions for unallocated LAE									3,441	6,753	(187)
Amortization of deferred charges									0	0	0
Amortization of fair value adjustments									168	(266)	(945)
Total changes in fair value - fair value option									0	0	0
Total net incurred losses and LAE									727,636	673,383	314,806
Other
Total
Net losses paid									11,822	4,092
Net change in case and LAE reserves									3,460	4,808
Net change in IBNR reserves									756	7,999
Increase (reduction) in estimates of net ultimate losses									16,038	16,899
Increase (reduction) in provisions for unallocated LAE									0	0
Amortization of deferred charges									0	0
Amortization of fair value adjustments									0	0
Total changes in fair value - fair value option									0	0
Total net incurred losses and LAE									$ 16,038	$ 16,899	$ 0