Schedule III - Supplementary Insurance Information (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
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net premiums earned									$ 847,532	$ 818,853	$ 738,531
Net investment income									93,269	91,884	95,662
Losses and loss adjustment expenses incurred related to current year, net of reinsurance	$ 233,680	$ 177,555	$ 184,435	$ 170,032	$ 179,056	$ 169,154	$ 184,543	$ 152,572	765,698	685,326	603,518
Losses and loss adjustment expenses incurred related to prior year, net of reinsurance	30,428	$ (15,941)	$ (15,995)	$ (10,277)	(24,967)	$ (21,549)	$ (22,815)	$ (22,786)	(11,783)	(92,116)	(134,360)
Paid losses and loss adjustment expenses, net of reinsurance									574,124	529,979	476,315
Underwriting, policy acquisition and operating expenses:									115,330	104,501	95,751
Other underwriting, policy acquisition and operating expenses									138,178	134,055	140,002
Net premiums written									842,725	834,914	764,018
Deferred policy acquisition costs	55,567				54,116				55,567	54,116	50,261
Reserve for losses and loss adjustment expenses	2,346,526				2,119,847				2,346,526	2,119,847	2,048,381
Unearned premiums	$ 413,086				$ 415,211				413,086	415,211	398,884
Specialty P&C
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net premiums earned									499,058	491,787	449,823
Losses and loss adjustment expenses incurred related to prior year, net of reinsurance									5,741	(77,085)	(119,293)
Net premiums written									495,750	494,148	466,621
Workers' Compensation Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net premiums earned									189,240	186,079	163,309
Losses and loss adjustment expenses incurred related to prior year, net of reinsurance									(7,801)	(8,051)	(5,742)
Net premiums written									182,233	195,350	173,566
Segregated Portfolio Cell Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net premiums earned									78,563	73,940	68,197
Net investment income									1,578	1,566	1,059
Losses and loss adjustment expenses incurred related to prior year, net of reinsurance									(10,134)	(8,967)	(8,513)
Net premiums written									77,639	75,547	68,862
Lloyd's Syndicates
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net premiums earned									80,671	67,047	57,202
Net investment income									4,551	3,358	1,736
Losses and loss adjustment expenses incurred related to prior year, net of reinsurance									411	1,987	(812)
Net premiums written									87,103	69,869	54,969
Corporate
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net investment income									87,140	86,960	92,867
Operating segments | Specialty P&C
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Losses and loss adjustment expenses incurred related to current year, net of reinsurance									526,744	461,516	404,543
Paid losses and loss adjustment expenses, net of reinsurance									382,845	354,221	320,776
Underwriting, policy acquisition and operating expenses:									56,605	52,253	47,615
Other underwriting, policy acquisition and operating expenses									63,705	60,166	60,357
Operating segments | Workers' Compensation Insurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Losses and loss adjustment expenses incurred related to current year, net of reinsurance									129,450	126,534	107,975
Paid losses and loss adjustment expenses, net of reinsurance									117,848	108,742	96,734
Underwriting, policy acquisition and operating expenses:									17,144	16,864	14,551
Other underwriting, policy acquisition and operating expenses									40,376	38,829	38,025
Operating segments | Segregated Portfolio Cell Reinsurance
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Losses and loss adjustment expenses incurred related to current year, net of reinsurance									62,546	47,693	45,968
Paid losses and loss adjustment expenses, net of reinsurance									37,034	29,320	28,761
Underwriting, policy acquisition and operating expenses:									21,717	21,039	19,927
Other underwriting, policy acquisition and operating expenses									2,543	1,387	837
Operating segments | Lloyd's Syndicates
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Losses and loss adjustment expenses incurred related to current year, net of reinsurance									46,958	49,583	45,032
Paid losses and loss adjustment expenses, net of reinsurance									36,593	37,496	29,926
Underwriting, policy acquisition and operating expenses:									21,392	15,913	15,194
Other underwriting, policy acquisition and operating expenses									13,319	15,773	11,769
Operating segments | Corporate
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Other underwriting, policy acquisition and operating expenses									19,146	18,767	29,275
Inter-segment Eliminations
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Paid losses and loss adjustment expenses, net of reinsurance									(196)	200	118
Underwriting, policy acquisition and operating expenses:									(1,528)	(1,568)	(1,536)
Other underwriting, policy acquisition and operating expenses									$ (911)	$ (867)	$ (261)