Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net premiums earned	$ 187,358	$ 203,855
Net investment income	15,017	20,830
Equity in earnings (loss) of unconsolidated subsidiaries	6,788	(1,562)
Net realized gains (losses)	8,849	(28,673)
Other income (expense)	2,005	2,251
Net losses and loss adjustment expenses	(149,785)	(164,832)
Underwriting, policy acquisition and operating expenses	(56,451)	(62,056)
SPC U.S. federal income taxes expense	(356)	(222)
SPC dividend (expense) income	(1,742)	508
Interest expense	(3,212)	(4,129)
Income tax benefit (expense)	(736)	12,076
Segment results	7,735	(21,954)
Net income (loss)	7,735	(21,954)
Depreciation and amortization, net of accretion	6,722	4,734
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	115,613	120,359
Workers' Compensation Insurance Segment
Segment Reporting Information [Line Items]
Net premiums earned	40,011	44,515
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	15,884	16,980
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	15,850	22,001
Operating segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	115,613	120,359
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	0	0
Other income (expense)	469	1,698
Net losses and loss adjustment expenses	(101,186)	(110,931)
Underwriting, policy acquisition and operating expenses	(26,346)	(29,585)
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment results	(11,450)	(18,459)
Depreciation and amortization, net of accretion	2,171	1,580
Operating segments | Workers' Compensation Insurance Segment
Segment Reporting Information [Line Items]
Net premiums earned	40,011	44,515
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	0	0
Other income (expense)	392	757
Net losses and loss adjustment expenses	(26,207)	(29,769)
Underwriting, policy acquisition and operating expenses	(12,286)	(14,164)
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment results	1,910	1,339
Depreciation and amortization, net of accretion	903	926
Operating segments | Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	15,884	16,980
Net investment income	221	254
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	987	(3,207)
Other income (expense)	1	136
Net losses and loss adjustment expenses	(9,425)	(9,352)
Underwriting, policy acquisition and operating expenses	(5,025)	(5,079)
SPC U.S. federal income taxes expense	(356)	(222)
SPC dividend (expense) income	(1,742)	508
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment results	545	18
Depreciation and amortization, net of accretion	316	69
Operating segments | Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	15,850	22,001
Net investment income	729	1,159
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	(115)	81
Other income (expense)	221	(232)
Net losses and loss adjustment expenses	(12,967)	(14,780)
Underwriting, policy acquisition and operating expenses	(6,591)	(9,142)
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	0	29
Segment results	(2,873)	(884)
Depreciation and amortization, net of accretion	15	9
Operating segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Net investment income	14,067	19,417
Equity in earnings (loss) of unconsolidated subsidiaries	6,788	(1,562)
Net realized gains (losses)	7,977	(25,547)
Other income (expense)	1,894	633
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	(7,175)	(4,827)
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	(3,212)	(4,129)
Income tax benefit (expense)	(736)	12,047
Segment results	19,603	(3,968)
Depreciation and amortization, net of accretion	3,317	2,150
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)	(972)	(741)
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	972	741
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment results	0	0
Depreciation and amortization, net of accretion	$ 0	$ 0