Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 207,293	$ 209,149	$ 411,149	$ 417,298
Net investment income	18,124	23,539	38,954	46,357
Equity in earnings (loss) of unconsolidated subsidiaries	(25,355)	(5,152)	(26,917)	(5,962)
Net realized gains (losses)	19,985	9,308	(8,688)	45,931
Other income (expense)	1,695	2,777	3,945	4,872
Net losses and loss adjustment expenses	(210,999)	(168,440)	(375,831)	(328,195)
Underwriting, policy acquisition and operating expenses	(58,692)	(62,708)	(120,746)	(124,100)
SPC U.S. federal income taxes	(480)	0	(702)	0
Segregated portfolio cells dividend expense (income)	(4,642)	7,033	(4,134)	2,246
Interest expense	(3,714)	(4,247)	(7,844)	(8,576)
Income tax benefit (expense)	38,686	277	50,761	(6,685)
Net income (loss)	(18,099)	11,536	(40,053)	43,186
Depreciation and amortization, net of accretion	5,085	4,617	9,824	9,653
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	127,096	126,011	247,456	250,079
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	42,406	46,574	86,921	92,512
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,748	19,284	33,728	38,787
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	21,043	17,280	43,044	35,920
Operating segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	127,096	126,011	247,456	250,079
Net investment income	0	0	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	0	0	0	0
Other income (expense)	1,092	1,470	2,788	2,680
Net losses and loss adjustment expenses	(159,559)	(106,017)	(270,491)	(213,675)
Underwriting, policy acquisition and operating expenses	(25,234)	(29,863)	(54,818)	(59,480)
SPC U.S. federal income taxes	0	0	0	0
Segregated portfolio cells dividend expense (income)	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Net income (loss)	(56,605)	(8,399)	(75,065)	(20,396)
Depreciation and amortization, net of accretion	1,680	1,623	3,260	3,359
Operating segments | Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	42,406	46,574	86,921	92,512
Net investment income	0	0	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	0	0	0	0
Other income (expense)	519	725	1,276	1,454
Net losses and loss adjustment expenses	(28,425)	(30,625)	(58,192)	(61,068)
Underwriting, policy acquisition and operating expenses	(13,456)	(14,368)	(27,622)	(28,559)
SPC U.S. federal income taxes	0	0	0	0
Segregated portfolio cells dividend expense (income)	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Net income (loss)	1,044	2,306	2,383	4,339
Depreciation and amortization, net of accretion	922	959	1,848	1,936
Operating segments | Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,748	19,284	33,728	38,787
Net investment income	305	368	559	815
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	2,606	(94)	(601)	2,047
Other income (expense)	55	135	191	221
Net losses and loss adjustment expenses	(7,680)	(19,973)	(17,032)	(30,719)
Underwriting, policy acquisition and operating expenses	(5,360)	(5,905)	(10,439)	(11,138)
SPC U.S. federal income taxes	(480)	0	(702)	0
Segregated portfolio cells dividend expense (income)	(4,642)	7,033	(4,134)	2,246
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Net income (loss)	1,552	848	1,570	2,259
Depreciation and amortization, net of accretion	91	(32)	160	14
Operating segments | Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	21,043	17,280	43,044	35,920
Net investment income	1,126	1,199	2,285	2,205
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	529	262	611	440
Other income (expense)	40	32	(192)	(114)
Net losses and loss adjustment expenses	(15,335)	(11,825)	(30,116)	(22,733)
Underwriting, policy acquisition and operating expenses	(7,293)	(7,564)	(16,434)	(16,033)
SPC U.S. federal income taxes	0	0	0	0
Segregated portfolio cells dividend expense (income)	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	304	29	0
Net income (loss)	110	(312)	(773)	(315)
Depreciation and amortization, net of accretion	14	(4)	23	(7)
Operating segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Net investment income	16,693	21,972	36,110	43,337
Equity in earnings (loss) of unconsolidated subsidiaries	(25,355)	(5,152)	(26,917)	(5,962)
Net realized gains (losses)	16,850	9,140	(8,698)	43,444
Other income (expense)	404	833	1,038	1,738
Net losses and loss adjustment expenses	0	0	0	0
Underwriting, policy acquisition and operating expenses	(7,764)	(5,426)	(12,589)	(9,997)
SPC U.S. federal income taxes	0	0	0	0
Segregated portfolio cells dividend expense (income)	0	0	0	0
Interest expense	(3,714)	(4,247)	(7,844)	(8,576)
Income tax benefit (expense)	38,686	(27)	50,732	(6,685)
Net income (loss)	35,800	17,093	31,832	57,299
Depreciation and amortization, net of accretion	2,378	2,071	4,533	4,351
Inter-segment Eliminations
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Net investment income	0	0	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	0	0	0	0
Other income (expense)	(415)	(418)	(1,156)	(1,107)
Net losses and loss adjustment expenses	0	0	0	0
Underwriting, policy acquisition and operating expenses	415	418	1,156	1,107
SPC U.S. federal income taxes	0	0	0	0
Segregated portfolio cells dividend expense (income)	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Net income (loss)	0	0	0	0
Depreciation and amortization, net of accretion	$ 0	$ 0	$ 0	$ 0