Shareholders' Equity (Roll Forward of AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 1,511,913	$ 1,523,002
OCI, before reclassifications, net of tax	(41,980)	25,553
Amounts reclassified from AOCI, net of tax	115	13
Net OCI, current period	(41,865)	25,566
Ending balance	1,427,509	1,561,896
Unrealized Investment Gains (Losses)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	36,577	(17,089)
OCI, before reclassifications, net of tax	(42,497)	25,477
Amounts reclassified from AOCI, net of tax	115	13
Net OCI, current period	(42,382)	25,490
Ending balance	(5,805)	8,401
Non-credit Impairments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	300	121
OCI, before reclassifications, net of tax	517	69
Amounts reclassified from AOCI, net of tax	0	0
Net OCI, current period	517	69
Ending balance	817	190
Unrecognized Change in Defined Benefit Plan Liabilities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	78	57
OCI, before reclassifications, net of tax	0	7
Amounts reclassified from AOCI, net of tax	0	0
Net OCI, current period	0	7
Ending balance	78	64
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	36,955	(16,911)
Ending balance	$ (4,910)	$ 8,655