Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net premiums earned	$ 194,559	$ 215,788	$ 605,708	$ 633,086
Net investment income	16,924	23,681	55,877	70,038
Equity in earnings (loss) of unconsolidated subsidiaries	4,853	(1,277)	(22,065)	(7,240)
Net realized gains (losses)	8,838	1,134	150	47,064
Other income (expense)	1,723	2,548	5,668	7,419
Net losses and loss adjustment expenses	(145,581)	(161,614)	(521,412)	(489,808)
Underwriting, policy acquisition and operating expenses	(59,433)	(61,861)	(180,178)	(185,960)
SPC U.S. federal income taxes	(871)	0	(1,573)	0
SPC dividend (expense) income	(3,854)	(3,621)	(7,988)	(1,375)
Interest expense	(3,881)	(4,274)	(11,725)	(12,850)
Income tax benefit (expense)	(2,141)	6,689	48,621	4
Segment results	11,136	17,193	(28,917)	60,378
Goodwill impairment	161,115	0	161,115	0
Net income (loss)	(149,979)	17,193	(190,032)	60,378
Depreciation and amortization, net of accretion	6,345	4,420	16,168	14,073
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	117,849	125,237	365,305	375,315
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	42,516	49,477	129,437	141,990
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,052	19,779	49,780	58,566
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	18,142	21,295	61,186	57,215
Operating segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	117,849	125,237	365,305	375,315
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)	726	1,858	3,515	4,536
Net losses and loss adjustment expenses	(102,951)	(107,573)	(373,442)	(321,248)
Underwriting, policy acquisition and operating expenses	(28,074)	(29,700)	(82,894)	(89,177)
SPC U.S. federal income taxes	0	0	0	0
SPC dividend (expense) income	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Segment results	(12,450)	(10,178)	(87,516)	(30,574)
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	2,669	1,568	5,930	4,927
Operating segments | Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	42,516	49,477	129,437	141,990
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)	441	494	1,717	1,948
Net losses and loss adjustment expenses	(26,455)	(32,356)	(84,648)	(93,424)
Underwriting, policy acquisition and operating expenses	(14,983)	(14,895)	(42,604)	(43,456)
SPC U.S. federal income taxes	0	0	0	0
SPC dividend (expense) income	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Segment results	1,519	2,720	3,902	7,058
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	923	957	2,771	2,893
Operating segments | Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	16,052	19,779	49,780	58,566
Net investment income	273	445	832	1,261
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	1,495	(98)	894	1,949
Other income (expense)	12	176	203	397
Net losses and loss adjustment expenses	(6,858)	(9,778)	(23,890)	(40,496)
Underwriting, policy acquisition and operating expenses	(5,036)	(5,951)	(15,474)	(17,091)
SPC U.S. federal income taxes	(871)	0	(1,573)	0
SPC dividend (expense) income	(3,854)	(3,621)	(7,988)	(1,375)
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Segment results	1,213	952	2,784	3,211
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	210	(59)	371	(45)
Operating segments | Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	18,142	21,295	61,186	57,215
Net investment income	951	1,077	3,236	3,282
Equity in earnings (loss) of unconsolidated subsidiaries	0	0	0	0
Net realized gains (losses)	489	285	1,100	725
Other income (expense)	411	(165)	219	(278)
Net losses and loss adjustment expenses	(9,317)	(11,907)	(39,432)	(34,640)
Underwriting, policy acquisition and operating expenses	(6,938)	(9,411)	(23,373)	(25,445)
SPC U.S. federal income taxes	0	0	0	0
SPC dividend (expense) income	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	161	29	161
Segment results	3,738	1,335	2,965	1,020
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	3	(1)	26	(8)
Operating segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0	0	0
Net investment income	15,700	22,159	51,809	65,495
Equity in earnings (loss) of unconsolidated subsidiaries	4,853	(1,277)	(22,065)	(7,240)
Net realized gains (losses)	6,854	947	(1,844)	44,390
Other income (expense)	775	963	1,813	2,701
Net losses and loss adjustment expenses	0	0	0	0
Underwriting, policy acquisition and operating expenses	(5,044)	(2,682)	(17,632)	(12,676)
SPC U.S. federal income taxes	0	0	0	0
SPC dividend (expense) income	0	0	0	0
Interest expense	(3,881)	(4,274)	(11,725)	(12,850)
Income tax benefit (expense)	(2,141)	6,528	48,592	(157)
Segment results	17,116	22,364	48,948	79,663
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	2,540	1,955	7,070	6,306
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)	(642)	(778)	(1,799)	(1,885)
Net losses and loss adjustment expenses	0	0	0	0
Underwriting, policy acquisition and operating expenses	642	778	1,799	1,885
SPC U.S. federal income taxes	0	0	0	0
SPC dividend (expense) income	0	0	0	0
Interest expense	0	0	0	0
Income tax benefit (expense)	0	0	0	0
Segment results	0	0	0	0
Goodwill impairment	0		0
Depreciation and amortization, net of accretion	0	$ 0	0	$ 0
Segment reconciling items
Segment Reporting Information [Line Items]
Goodwill impairment	$ 161,115		$ 161,115