Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Net premiums earned	$ 182,903	$ 177,579
Net investment income	23,186	25,440
Equity in earnings (loss) of unconsolidated subsidiaries	1,808	(3,634)
Net realized gains (losses)	13,280	(8,352)
Other income	1,821	2,354
Net losses and loss adjustment expenses	(119,151)	(110,955)
Underwriting, policy acquisition and operating expenses	(57,108)	(56,889)
Segregated portfolio cells dividend (expense) income	(2,375)	(1,176)
Interest expense	(4,133)	(3,686)
Income tax benefit (expense)	1,224	(1,364)
Net income	41,455	19,317
Depreciation and amortization, net of accretion	7,803	8,630
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segregated portfolio cells dividend (expense) income		(463)
Operating Segments [Member] | Specialty P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned	113,058	110,755
Other income	1,198	1,276
Net losses and loss adjustment expenses	(74,994)	(71,174)
Underwriting, policy acquisition and operating expenses	(25,977)	(25,054)
Segregated portfolio cells dividend (expense) income	28
Net income	13,313	15,803
Depreciation and amortization, net of accretion	1,911	2,011
Operating Segments [Member] | Workers Compensation [Member]
Segment Reporting Information [Line Items]
Net premiums earned	55,283	54,383
Other income	145	471
Net losses and loss adjustment expenses	(34,650)	(35,027)
Underwriting, policy acquisition and operating expenses	(16,691)	(17,829)
Segregated portfolio cells dividend (expense) income	(1,174)	(713)
Net income	2,913	1,285
Depreciation and amortization, net of accretion	837	1,404
Operating Segments [Member] | Lloyds Syndicate [Member]
Segment Reporting Information [Line Items]
Net premiums earned	14,562	12,441
Net investment income	372	315
Net realized gains (losses)	27	(5)
Other income	391	252
Net losses and loss adjustment expenses	(9,507)	(6,189)
Underwriting, policy acquisition and operating expenses	(6,211)	(5,167)
Income tax benefit (expense)	7	(84)
Net income	(359)	1,563
Depreciation and amortization, net of accretion	(3)	66
Operating Segments [Member] | Corporate [Member]
Segment Reporting Information [Line Items]
Net investment income	22,814	25,125
Equity in earnings (loss) of unconsolidated subsidiaries	1,808	(3,634)
Net realized gains (losses)	13,253	(8,347)
Other income	173	458
Underwriting, policy acquisition and operating expenses	(8,315)	(7,507)
Segregated portfolio cells dividend (expense) income	(1,229)
Interest expense	(4,133)	(3,686)
Income tax benefit (expense)	1,217	(1,280)
Net income	25,588	666
Depreciation and amortization, net of accretion	5,058	5,149
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Other income	(86)	(103)
Net losses and loss adjustment expenses	0	1,435
Underwriting, policy acquisition and operating expenses	86	(1,332)
Net income	0	0
Depreciation and amortization, net of accretion	$ 0	$ 0