Shareholders' Equity (Roll Forward of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,321,316	$ 1,427,509	$ 1,349,210	$ 1,511,913
OCI, before reclassifications, net of tax	15,926	68,972	(14,489)	26,991
Amounts reclassified from AOCI, net of tax	(4,376)	(2,932)	(7,666)	(2,816)
Net OCI, current period	11,550	66,040	(22,155)	24,175
Ending balance	1,424,040	1,474,390	1,424,040	1,474,390
Unrealized Investment Gains (Losses)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	41,626	(4,015)	75,388	37,333
OCI, before reclassifications, net of tax	15,918	69,159	(14,497)	27,695
Amounts reclassified from AOCI, net of tax	(4,376)	(3,232)	(7,723)	(3,116)
Net OCI, current period	11,542	65,927	(22,220)	24,579
Ending balance	53,168	61,912	53,168	61,912
Non-credit Impairments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0	(817)	(57)	(300)
OCI, before reclassifications, net of tax	0	(187)	0	(704)
Amounts reclassified from AOCI, net of tax	0	300	57	300
Net OCI, current period	0	113	57	(404)
Ending balance	0	(704)	0	(704)
Unrecognized Change in Defined Benefit Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(104)	(78)	(104)	(78)
OCI, before reclassifications, net of tax	8	0	8	0
Amounts reclassified from AOCI, net of tax	0	0	0	0
Net OCI, current period	8	0	8	0
Ending balance	(96)	(78)	(96)	(78)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	41,522	(4,910)	75,227	36,955
Ending balance	$ 53,072	$ 61,130	$ 53,072	$ 61,130