Segment Information (Financial Data by Segment) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Net premiums earned	$ 265,711	$ 187,358
Net investment income	20,443	15,017
Equity in earnings (loss) of unconsolidated subsidiaries	7,620	6,788
Net investment gains (losses)	(13,506)	8,849
Other income (expense)	2,804	2,005
Net losses and loss adjustment expenses	(209,423)	(149,785)
Underwriting, policy acquisition and operating expenses	(70,599)	(56,451)
SPC U.S. federal income taxes expense	(642)	(356)
SPC dividend (expense) income	(2,367)	(1,742)
Interest expense	(4,441)	(3,212)
Income tax benefit (expense)	1,770	(736)
Segment results	(2,630)	7,735
Net income (loss)	(3,560)	7,735
Depreciation and amortization, net of accretion	10,350	6,722
Income tax expense (benefit)	(2,017)	736
NORCAL Group
Segment Reporting Information [Line Items]
Net losses and loss adjustment expenses	7,300
Underwriting, policy acquisition and operating expenses	(7,300)
Transaction-related costs	(1,200)
Transaction-related costs	1,200
Income tax expense (benefit)	300
Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	197,967	115,613
Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	40,684	40,011
Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	19,314	15,884
Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	7,746	15,850
Operating segments | Specialty P&C
Segment Reporting Information [Line Items]
Net premiums earned	197,967	115,613
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net investment gains (losses)	0	0
Other income (expense)	1,019	469
Net losses and loss adjustment expenses	(165,958)	(101,186)
Underwriting, policy acquisition and operating expenses	(42,878)	(26,346)
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	(9,850)	(11,450)
Depreciation and amortization, net of accretion	2,592	2,171
Operating segments | Workers' Compensation Insurance
Segment Reporting Information [Line Items]
Net premiums earned	40,684	40,011
Net investment income	0	0
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net investment gains (losses)	0	0
Other income (expense)	682	392
Net losses and loss adjustment expenses	(27,211)	(26,207)
Underwriting, policy acquisition and operating expenses	(13,001)	(12,286)
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	1,154	1,910
Depreciation and amortization, net of accretion	874	903
Operating segments | Segregated Portfolio Cell Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned	19,314	15,884
Net investment income	112	221
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net investment gains (losses)	(711)	987
Other income (expense)	1	1
Net losses and loss adjustment expenses	(11,491)	(9,425)
Underwriting, policy acquisition and operating expenses	(4,369)	(5,025)
SPC U.S. federal income taxes expense	(642)	(356)
SPC dividend (expense) income	(2,367)	(1,742)
Interest expense	0	0
Income tax benefit (expense)	0	0
Segment results	(153)	545
Depreciation and amortization, net of accretion	370	316
Operating segments | Lloyd's Syndicates
Segment Reporting Information [Line Items]
Net premiums earned	7,746	15,850
Net investment income	211	729
Equity in earnings (loss) of unconsolidated subsidiaries	0	0
Net investment gains (losses)	(399)	(115)
Other income (expense)	134	221
Net losses and loss adjustment expenses	(4,763)	(12,967)
Underwriting, policy acquisition and operating expenses	(2,709)	(6,591)
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	220	(2,873)
Depreciation and amortization, net of accretion	14	15
Operating segments | Corporate
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Net investment income	20,120	14,067
Equity in earnings (loss) of unconsolidated subsidiaries	7,620	6,788
Net investment gains (losses)	(12,396)	7,977
Other income (expense)	2,065	1,894
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	(8,739)	(7,175)
SPC U.S. federal income taxes expense	0	0
SPC dividend (expense) income	0	0
Interest expense	(4,441)	(3,212)
Income tax benefit (expense)	1,770	(736)
Segment results	5,999	19,603
Depreciation and amortization, net of accretion	6,500	3,317
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 investment gains (losses)	0	0
Other income (expense)	(1,097)	(972)
Net losses and loss adjustment expenses	0	0
Underwriting, policy acquisition and operating expenses	1,097	972
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
Segment reconciling items
Segment Reporting Information [Line Items]
Transaction-related costs	(930)
Transaction-related costs	$ 930