Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 2,340,501	$ 2,308,798	$ 2,342,176	$ 2,247,573
Ending Balance	2,390,261	2,324,016	2,390,261	2,324,016
Unrealized Gains (Losses) on Investment Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	65,930	2,368	14,864	(44,654)
Other comprehensive income before reclassifications	4,333	18,029	56,186	63,244
Amounts reclassified from accumulated other comprehensive income	(1)	(3,498)	(2,377)	(3,686)
Ending Balance	71,190	20,329	71,190	20,329
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	0	(2)	0	680
Other comprehensive income before reclassifications	0	0	0	(682)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Ending Balance	0	(2)	0	(2)
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(14,491)	(14,523)	(15,001)	(15,089)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	255	277	765	843
Ending Balance	(14,236)	(14,246)	(14,236)	(14,246)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	51,439	(12,157)	(137)	(59,063)
Other comprehensive income before reclassifications	4,333	18,029	56,186	62,562
Amounts reclassified from accumulated other comprehensive income	254	(3,221)	(1,612)	(2,843)
Ending Balance	56,954	6,081	56,954	6,081
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	928	3,430	2,517	5,425
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Investment Securities
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	928	3,430	2,517	5,425
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	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	$ 0	$ 0