Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 203,855	$ 208,149
Net investment income	20,830	22,818
Equity in earnings (loss) of unconsolidated subsidiaries	(1,562)	(810)
Net realized gains (losses)	(28,673)	36,623
Other income (expense)	2,251	2,095
Net losses and loss adjustment expenses	(164,832)	(159,755)
Underwriting, policy acquisition and operating expenses	(62,056)	(61,392)
SPC U.S. federal income taxes	(222)	0
Segregated portfolio cells dividend expense (income)	508	(4,787)
Interest expense	(4,129)	(4,330)
Income tax benefit (expense)	12,076	(6,961)
Net income (loss)	(21,954)	31,650
Depreciation and amortization, net of accretion	4,734	5,036
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	120,359	124,067
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	44,515	45,939
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,980	19,502
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	22,001	18,641
Operating segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	120,359	124,067
Other income (expense)	1,698	1,209
Net losses and loss adjustment expenses	(110,931)	(107,658)
Underwriting, policy acquisition and operating expenses	(29,585)	(29,615)
SPC U.S. federal income taxes	0	0
Net income (loss)	(18,459)	(11,997)
Depreciation and amortization, net of accretion	1,580	1,736
Operating segments | Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	44,515	45,939
Other income (expense)	757	729
Net losses and loss adjustment expenses	(29,769)	(30,443)
Underwriting, policy acquisition and operating expenses	(14,164)	(14,192)
SPC U.S. federal income taxes	0	0
Net income (loss)	1,339	2,033
Depreciation and amortization, net of accretion	926	977
Operating segments | Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,980	19,502
Net investment income	254	448
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	(3,207)	2,141
Other income (expense)	136	87
Net losses and loss adjustment expenses	(9,352)	(10,745)
Underwriting, policy acquisition and operating expenses	(5,079)	(5,235)
SPC U.S. federal income taxes	(222)	0
Segregated portfolio cells dividend expense (income)	508	(4,787)
Net income (loss)	18	1,411
Depreciation and amortization, net of accretion	69	46
Operating segments | Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	22,001	18,641
Net investment income	1,159	1,006
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net realized gains (losses)	81	178
Other income (expense)	(232)	(146)
Net losses and loss adjustment expenses	(14,780)	(10,909)
Underwriting, policy acquisition and operating expenses	(9,142)	(8,469)
SPC U.S. federal income taxes	0	0
Income tax benefit (expense)	29	(304)
Net income (loss)	(884)	(3)
Depreciation and amortization, net of accretion	9	(3)
Operating segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Net investment income	19,417	21,364
Equity in earnings (loss) of unconsolidated subsidiaries	(1,562)	(810)
Net realized gains (losses)	(25,547)	34,304
Other income (expense)	633	905
Underwriting, policy acquisition and operating expenses	(4,827)	(4,570)
SPC U.S. federal income taxes	0	0
Segregated portfolio cells dividend expense (income)	0	0
Interest expense	(4,129)	(4,330)
Income tax benefit (expense)	12,047	(6,657)
Net income (loss)	(3,968)	40,206
Depreciation and amortization, net of accretion	2,150	2,280
Inter-segment Eliminations
Segment Reporting Information [Line Items]
Other income (expense)	(741)	(689)
Underwriting, policy acquisition and operating expenses	741	689
SPC U.S. federal income taxes		0
Interest expense		0
Net income (loss)	0	0
Depreciation and amortization, net of accretion	$ 0	$ 0