Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	$ 153,505	$ 121,101	$ 64,984
Reserves for Losses and Loss Adjustment Expenses	10,429,238	9,409,504	7,398,088
Unearned Premiums	746,775	842,618	583,197
Policy Benefits for Life and Annuity Contracts	0	105,080	117,207
Net Premiums Earned	1,154,861	895,575	613,121
Net Investment Income	321,276	270,671	208,789
Losses and Loss Expenses and Policy Benefits	872,666	455,028	197,566
Amortization of Deferred Acquisition Costs	305,951	192,790	96,906
Other Operating Expenses	473,086	407,375	435,985
Net Premiums Written	900,959	981,900	610,390
Operating Segments | Non-life Run-off
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	41,753	4,378	655
Reserves for Losses and Loss Adjustment Expenses	8,295,361	7,540,662	5,949,472
Unearned Premiums	129,715	136,023	14,275
Policy Benefits for Life and Annuity Contracts	0	0	0
Net Premiums Earned	168,496	9,427	14,162
Net Investment Income	275,236	226,287	166,678
Losses and Loss Expenses and Policy Benefits	51,625	(306,067)	(190,674)
Amortization of Deferred Acquisition Costs	73,642	4,006	328
Other Operating Expenses	270,445	197,824	233,827
Net Premiums Written	(25,338)	(9,217)	6,482
Operating Segments | Atrium
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	22,184	20,355	18,385
Reserves for Losses and Loss Adjustment Expenses	231,672	241,284	240,873
Unearned Premiums	80,863	70,429	64,877
Policy Benefits for Life and Annuity Contracts	0	0	0
Net Premiums Earned	164,059	146,315	134,747
Net Investment Income	7,049	5,686	4,218
Losses and Loss Expenses and Policy Benefits	77,276	69,810	69,419
Amortization of Deferred Acquisition Costs	56,956	50,646	47,688
Other Operating Expenses	28,277	24,698	29,586
Net Premiums Written	172,356	153,488	134,214
Operating Segments | StarStone
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	89,180	96,004	45,944
Reserves for Losses and Loss Adjustment Expenses	1,879,128	1,608,697	1,207,743
Unearned Premiums	518,199	619,164	504,045
Policy Benefits for Life and Annuity Contracts	0	0	0
Net Premiums Earned	801,926	714,959	459,403
Net Investment Income	47,401	35,973	27,706
Losses and Loss Expenses and Policy Benefits	727,636	673,383	314,806
Amortization of Deferred Acquisition Costs	174,711	135,452	48,012
Other Operating Expenses	128,419	156,726	135,558
Net Premiums Written	735,429	805,562	464,901
Other
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Acquisition Costs	388	364	0
Reserves for Losses and Loss Adjustment Expenses	23,077	18,861	0
Unearned Premiums	17,998	17,002	0
Policy Benefits for Life and Annuity Contracts	0	105,080	117,207
Net Premiums Earned	20,380	24,874	4,809
Net Investment Income	(8,410)	2,725	10,187
Losses and Loss Expenses and Policy Benefits	16,129	17,902	4,015
Amortization of Deferred Acquisition Costs	642	2,686	878
Other Operating Expenses	45,945	28,127	37,014
Net Premiums Written	$ 18,512	$ 32,067	$ 4,793