Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 2,342,176	$ 2,247,573
Ending Balance	2,285,748	2,301,019
Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	14,864	(44,654)
Other comprehensive income before reclassifications	17,431	20,298
Amounts reclassified from accumulated other comprehensive income	(36)	(51)
Ending Balance	33,054	(23,433)
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	0	680
Other comprehensive income before reclassifications	0	(82)
Amounts reclassified from accumulated other comprehensive income	0	0
Ending Balance	0	598
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(15,001)	(15,089)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	255	291
Ending Balance	(14,746)	(14,798)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(137)	(59,063)
Other comprehensive income before reclassifications	17,431	20,216
Amounts reclassified from accumulated other comprehensive income	219	240
Ending Balance	18,308	(37,633)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	795	974
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Investment Securities Not Other-Than-Temporarily Impaired
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	795	974
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	0	0
Reclassification out of Accumulated Other Comprehensive Income | Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	$ 0	$ 0