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 Policy Acquisition Costs	$ 62,006	$ 54,450	$ 72,365
Reserve for Losses and Loss Adjustment Expenses	2,045,506	1,661,459	1,292,349
Unearned Premiums	524,377	386,473	418,114
Net Earned Premiums	823,746	815,398	741,109
Net Investment Income	75,652	61,256	61,119
Losses and Loss Adjustment Expenses	672,102	600,276	555,377
Amortization of Policy Acquisition Costs	83,893	111,103	116,001
Other Operating Expenses	170,908	201,035	196,993
Net Written Premiums	896,150	762,672	766,626
Excess and Surplus Lines
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	24,428	16,736	17,050
Reserve for Losses and Loss Adjustment Expenses	1,245,581	960,562	759,043
Unearned Premiums	267,924	175,231	149,047
Net Earned Premiums	625,528	555,684	463,521
Net Investment Income	21,358	14,456	15,014
Losses and Loss Adjustment Expenses	528,133	437,904	371,717
Amortization of Policy Acquisition Costs	49,720	48,299	45,158
Other Operating Expenses	87,326	88,865	79,136
Net Written Premiums	685,814	571,098	469,891
Specialty Admitted Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	(2,902)	(1,829)	(725)
Reserve for Losses and Loss Adjustment Expenses	511,027	426,315	271,446
Unearned Premiums	115,606	84,659	73,649
Net Earned Premiums	54,338	55,146	68,110
Net Investment Income	3,802	3,262	2,532
Losses and Loss Adjustment Expenses	34,860	32,623	44,863
Amortization of Policy Acquisition Costs	(3,560)	(1,792)	5,605
Other Operating Expenses	13,986	15,551	20,081
Net Written Premiums	58,637	55,840	60,957
Casualty Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	40,480	39,543	56,040
Reserve for Losses and Loss Adjustment Expenses	288,898	274,582	261,860
Unearned Premiums	140,847	126,583	195,418
Net Earned Premiums	143,880	204,568	209,478
Net Investment Income	46,325	38,838	31,507
Losses and Loss Adjustment Expenses	109,109	129,749	138,797
Amortization of Policy Acquisition Costs	37,733	64,596	65,238
Other Operating Expenses	41,932	69,716	72,446
Net Written Premiums	151,699	135,734	235,778
Corporate and other segment
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Deferred Policy Acquisition Costs	0	0	0
Reserve for Losses and Loss Adjustment Expenses	0	0	0
Unearned Premiums	0	0	0
Net Earned Premiums	0	0	0
Net Investment Income	4,167	4,700	12,066
Losses and Loss Adjustment Expenses	0	0	0
Amortization of Policy Acquisition Costs	0	0	0
Other Operating Expenses	27,664	26,903	25,330
Net Written Premiums	$ 0	$ 0	$ 0