Shareholders' Equity (Roll Forward of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,474,390	$ 1,581,556	$ 1,511,913	$ 1,523,002
OCI, before reclassifications, net of tax	9,483	7,800	36,990	57,196
Amounts reclassified from AOCI, net of tax	(2,328)	(395)	(5,660)	(1,151)
Net OCI, current period	7,155	7,405	31,330	56,045
Ending balance	1,329,893	1,589,458	1,329,893	1,589,458
Unrealized Investment Gains (Losses)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	61,912	31,988	37,333	(16,733)
OCI, before reclassifications, net of tax	9,505	7,834	37,199	57,311
Amounts reclassified from AOCI, net of tax	(2,975)	(395)	(6,090)	(1,151)
Net OCI, current period	6,530	7,439	31,109	56,160
Ending balance	68,442	39,427	68,442	39,427
Non-credit Impairments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(704)	(190)	(300)	(121)
OCI, before reclassifications, net of tax	0	(29)	(187)	(98)
Amounts reclassified from AOCI, net of tax	647	0	430	0
Net OCI, current period	647	(29)	243	(98)
Ending balance	(57)	(219)	(57)	(219)
Unrecognized Change in Defined Benefit Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(78)	(69)	(78)	(57)
OCI, before reclassifications, net of tax	(22)	(5)	(22)	(17)
Amounts reclassified from AOCI, net of tax	0	0	0	0
Net OCI, current period	(22)	(5)	(22)	(17)
Ending balance	(100)	(74)	(100)	(74)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	61,130	31,729	36,955	(16,911)
Ending balance	$ 68,285	$ 39,134	$ 68,285	$ 39,134