Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net premiums earned	$ 187,159	$ 182,903
Net investment income	22,027	23,186
Equity in earnings (loss) of unconsolidated subsidiaries	1,640	1,808
Net realized gains (losses)	(12,517)	13,280
Other income (expense)	2,723	1,821
Net losses and loss adjustment expenses	(129,786)	(119,151)
Underwriting, policy acquisition and operating expenses	(57,360)	(57,108)
Segregated portfolio cells dividend (expense) income	(1,747)	(2,375)
Interest expense	(3,705)	(4,133)
Income tax benefit (expense)	3,422	1,224
Segment operating results	11,856	41,455
Depreciation and amortization, net of accretion	6,079	7,803
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	116,276	113,058
Workers' Compensation
Segment Reporting Information [Line Items]
Net premiums earned	58,407	55,283
Lloyd's Syndicate
Segment Reporting Information [Line Items]
Net premiums earned	12,476	14,562
Operating Segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	116,276	113,058
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	0	0
Other income (expense)	1,256	1,198
Net losses and loss adjustment expenses	(84,585)	(74,994)
Underwriting, policy acquisition and operating expenses	(28,276)	(25,977)
Segregated portfolio cells dividend (expense) income	30	28
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment operating results	4,701	13,313
Depreciation and amortization, net of accretion	1,867	1,911
Operating Segments | Workers' Compensation
Segment Reporting Information [Line Items]
Net premiums earned	58,407	55,283
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	0	0
Other income (expense)	366	145
Net losses and loss adjustment expenses	(36,715)	(34,650)
Underwriting, policy acquisition and operating expenses	(17,333)	(16,691)
Segregated portfolio cells dividend (expense) income	(1,894)	(1,174)
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment operating results	2,831	2,913
Depreciation and amortization, net of accretion	956	837
Operating Segments | Lloyd's Syndicate
Segment Reporting Information [Line Items]
Net premiums earned	12,476	14,562
Net investment income	751	372
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	(54)	27
Other income (expense)	331	391
Net losses and loss adjustment expenses	(8,486)	(9,507)
Underwriting, policy acquisition and operating expenses	(7,246)	(6,211)
Segregated portfolio cells dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	(6)	7
Segment operating results	(2,234)	(359)
Depreciation and amortization, net of accretion	(1)	(3)
Operating Segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Net investment income	21,276	22,814
Equity in earnings (loss) of unconsolidated subsidiaries	1,640	1,808
Net realized gains (losses)	(12,463)	13,253
Other income (expense)	943	173
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	(4,678)	(8,315)
Segregated portfolio cells dividend (expense) income	117	(1,229)
Interest expense	(3,705)	(4,133)
Income tax benefit (expense)	3,428	1,217
Segment operating results	6,558	25,588
Depreciation and amortization, net of accretion	3,257	5,058
Inter-segment Eliminations
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	0	0
Other income (expense)	(173)	(86)
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	173	86
Segregated portfolio cells dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment operating results	0	0
Depreciation and amortization, net of accretion	$ 0	$ 0