Losses and Loss Adjustment Expenses - Breakdown of Gross and Net Losses and LAE Reserves by Major Category (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gross
OLR	$ 5,394,529	$ 5,168,900
IBNR	5,057,868	4,468,991
Fair value adjustments - acquired companies	(167,511)	(214,518)
Fair value adjustments - fair value option	(217,933)	(374,752)
Total gross liability for unpaid losses and LAE	10,429,238	9,409,504	$ 7,398,088	$ 5,987,867
Net
ULAE	362,285	360,883
Total	8,047,055	7,455,078	5,447,863	4,505,123
Non-life Run-off
Gross
OLR	4,407,082	4,271,769
IBNR	3,945,407	3,527,767
Total	8,352,489	7,799,536
Fair value adjustments - acquired companies	(170,689)	(217,527)
Fair value adjustments - fair value option	(217,933)	(374,752)
Deferred charge on retroactive reinsurance	0	0
ULAE	331,494	333,405
Total gross liability for unpaid losses and LAE	8,295,361	7,540,662	5,949,472	4,716,363
Net
OLR	3,434,789	3,369,997
IBNR	3,272,348	2,918,333
Total Net Reserves	6,707,137	6,288,330
Fair value adjustments	(157,036)	(203,183)
Fair value adjustments - fair value option	(129,848)	(244,013)
Deferred charge on retroactive reinsurance	(272,462)	(86,585)
ULAE	331,494	333,405
Total	6,479,285	6,087,954	4,491,795	3,620,859
Non-life Run-off | Asbestos
Gross
OLR	542,681	341,544
IBNR	1,373,678	1,275,476
Total	1,916,359	1,617,020
Net
OLR	490,117	321,356
IBNR	1,271,982	1,171,754
Total Net Reserves	1,762,099	1,493,110
Non-life Run-off | Environmental
Gross
OLR	187,165	96,665
IBNR	156,121	126,035
Total	343,286	222,700
Net
OLR	173,878	93,095
IBNR	142,351	117,384
Total Net Reserves	316,229	210,479
Non-life Run-off | General casualty
Gross
OLR	501,863	500,033
IBNR	489,129	379,484
Total	990,992	879,517
Net
OLR	399,396	416,097
IBNR	421,426	298,612
Total Net Reserves	820,822	714,709
Non-life Run-off | Workers' compensation/personal accident
Gross
OLR	1,270,530	1,454,178
IBNR	977,808	832,615
Total	2,248,338	2,286,793
Net
OLR	963,578	1,115,116
IBNR	751,074	537,782
Total Net Reserves	1,714,652	1,652,898
Non-life Run-off | Marine, aviation and transit
Gross
OLR	290,067	301,783
IBNR	121,577	72,888
Total	411,644	374,671
Net
OLR	244,611	227,994
IBNR	100,135	78,023
Total Net Reserves	344,746	306,017
Non-life Run-off | Construction defect
Gross
OLR	29,772	20,712
IBNR	98,312	99,288
Total	128,084	120,000
Net
OLR	29,245	19,310
IBNR	94,888	94,736
Total Net Reserves	124,133	114,046
Non-life Run-off | Professional indemnity/Directors & Officers
Gross
OLR	693,760	603,665
IBNR	265,490	216,839
Total	959,250	820,504
Net
OLR	485,478	426,020
IBNR	170,926	166,898
Total Net Reserves	656,404	592,918
Non-life Run-off | Motor
Gross
OLR	480,668	564,307
IBNR	233,806	321,992
Total	714,474	886,299
Net
OLR	317,829	414,847
IBNR	165,543	304,874
Total Net Reserves	483,372	719,721
Non-life Run-off | Property
Gross
OLR	140,620	168,267
IBNR	63,604	37,631
Total	204,224	205,898
Net
OLR	122,010	160,873
IBNR	56,450	36,817
Total Net Reserves	178,460	197,690
Non-life Run-off | Other
Gross
OLR	269,956	220,615
IBNR	165,882	165,519
Total	435,838	386,134
Net
OLR	208,647	175,289
IBNR	97,573	111,453
Total Net Reserves	306,220	286,742
Atrium
Gross
OLR	89,141	94,885
IBNR	136,543	140,521
Total	225,684	235,406
Fair value adjustments - acquired companies	3,700	3,476
ULAE	2,288	2,402
Total gross liability for unpaid losses and LAE	231,672	241,284	240,873	212,122
Net
OLR	80,130	75,995
IBNR	117,257	121,272
Total Net Reserves	197,387	197,267
Fair value adjustments	3,181	2,847
ULAE	2,288	2,402
Total	202,856	202,516	200,342	182,113
Atrium | Marine, aviation and transit
Gross
OLR	24,668	32,999
IBNR	34,156	36,011
Total	58,824	69,010
Net
OLR	21,012	21,460
IBNR	24,829	24,207
Total Net Reserves	45,841	45,667
Atrium | Binding Authorities
Gross
OLR	31,507	28,512
IBNR	54,039	59,302
Total	85,546	87,814
Net
OLR	29,590	26,601
IBNR	51,984	57,016
Total Net Reserves	81,574	83,617
Atrium | Reinsurance
Gross
OLR	18,385	18,547
IBNR	29,533	27,653
Total	47,918	46,200
Net
OLR	16,209	15,180
IBNR	23,338	24,823
Total Net Reserves	39,547	40,003
Atrium | Accident and Health
Gross
OLR	5,460	4,972
IBNR	7,880	6,348
Total	13,340	11,320
Net
OLR	4,735	4,225
IBNR	7,469	5,837
Total Net Reserves	12,204	10,062
Atrium | Non-Marine Direct and Facultative
Gross
OLR	9,121	9,855
IBNR	10,935	11,207
Total	20,056	21,062
Net
OLR	8,584	8,529
IBNR	9,637	9,389
Total Net Reserves	18,221	17,918
StarStone
Gross
OLR	888,794	796,194
IBNR	962,353	787,894
Total	1,851,147	1,584,088
Fair value adjustments - acquired companies	(522)	(467)
ULAE	28,503	25,076
Total gross liability for unpaid losses and LAE	1,879,128	1,608,697	1,207,743	1,059,382
Net
OLR	560,784	533,129
IBNR	750,950	586,110
Total Net Reserves	1,311,734	1,119,239
Fair value adjustments	1,600	1,432
ULAE	28,503	25,076
Total	1,341,837	1,145,747	$ 755,726	$ 702,151
StarStone | General casualty
Gross
OLR	191,317	177,432
IBNR	481,473	331,432
Total	672,790	508,864
Net
OLR	156,086	137,828
IBNR	422,186	282,026
Total Net Reserves	578,272	419,854
StarStone | Workers' compensation/personal accident
Gross
OLR	55,902	49,373
IBNR	122,047	110,082
Total	177,949	159,455
Net
OLR	36,673	33,759
IBNR	74,661	68,969
Total Net Reserves	111,334	102,728
StarStone | Property
Gross
OLR	367,258	317,102
IBNR	152,514	123,511
Total	519,772	440,613
Net
OLR	158,156	151,774
IBNR	92,655	65,522
Total Net Reserves	250,811	217,296
StarStone | Marine
Gross
OLR	195,068	185,084
IBNR	173,319	182,453
Total	368,387	367,537
Net
OLR	162,073	163,889
IBNR	138,586	133,426
Total Net Reserves	300,659	297,315
StarStone | Aerospace
Gross
OLR	79,249	67,203
IBNR	33,000	40,416
Total	112,249	107,619
Net
OLR	47,796	45,879
IBNR	22,862	36,167
Total Net Reserves	$ 70,658	$ 82,046