SCHEDULE IV - Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 1,305,948	$ 1,027,222	$ 843,719
Ceded to Other Companies Written Premiums	574,585	404,101	315,279
Assumed from Other Companies Written Premiums	164,787	139,551	238,186
Net written premiums	$ 896,150	$ 762,672	$ 766,626
Percentage of Amount Assumed to Net	18.40%	18.30%	31.10%
Excess and Surplus Lines
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 922,320	$ 656,538	$ 530,077
Ceded to Other Companies Written Premiums	236,506	85,440	60,229
Assumed from Other Companies Written Premiums	0	0	43
Net written premiums	$ 685,814	$ 571,098	$ 469,891
Percentage of Amount Assumed to Net	0.00%	0.00%	0.00%
Specialty Admitted Insurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 383,628	$ 370,684	$ 313,642
Ceded to Other Companies Written Premiums	329,005	318,506	255,473
Assumed from Other Companies Written Premiums	4,014	3,662	2,788
Net written premiums	$ 58,637	$ 55,840	$ 60,957
Percentage of Amount Assumed to Net	6.80%	6.60%	4.60%
Casualty Reinsurance
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Written Premiums	$ 0	$ 0	$ 0
Ceded to Other Companies Written Premiums	9,074	155	(423)
Assumed from Other Companies Written Premiums	160,773	135,889	235,355
Net written premiums	$ 151,699	$ 135,734	$ 235,778
Percentage of Amount Assumed to Net	106.00%	100.10%	99.80%