Shareholders' Equity (Roll Forward of AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,349,210	$ 1,511,913	$ 1,523,002
OCI, before reclassifications, net of tax	(48,836)	46,170	55,841
Amounts reclassified from AOCI, net of tax	(10,107)	(7,898)	(1,975)
Net OCI, current period	(58,943)	38,272	53,866
Ending balance	1,428,387	1,349,210	1,511,913
Net actuarial gain	1,533
Unrealized Investment Gains (Losses)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	75,388	37,333	(16,733)
OCI, before reclassifications, net of tax	(50,242)	46,383	56,041
Amounts reclassified from AOCI, net of tax	(10,217)	(8,328)	(1,975)
Net OCI, current period	(60,459)	38,055	54,066
Ending balance	14,929	75,388	37,333
Non-credit Impairments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(57)	(300)	(121)
OCI, before reclassifications, net of tax	0	(187)	(179)
Amounts reclassified from AOCI, net of tax	57	430	0
Net OCI, current period	57	243	(179)
Ending balance	0	(57)	(300)
Unrecognized Change in Defined Benefit Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(104)	(78)	(57)
OCI, before reclassifications, net of tax	1,406	(26)	(21)
Amounts reclassified from AOCI, net of tax	53	0	0
Net OCI, current period	1,459	(26)	(21)
Ending balance	1,355	(104)	(78)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	75,227	36,955	(16,911)
Ending balance	16,284	$ 75,227	$ 36,955
Accumulated Other Comprehensive Income (Loss) | NORCAL Group
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net actuarial gain	$ 1,200