Accumulated Other Comprehensive Income Components of Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 2,285,748	$ 2,301,019	$ 2,342,176	$ 2,247,573
Ending Balance	2,340,501	2,308,798	2,340,501	2,308,798
Unrealized Gains (Losses) on Investment Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	33,054	(23,433)	14,864	(44,654)
Other comprehensive income before reclassifications	34,424	24,917	51,853	45,215
Amounts reclassified from accumulated other comprehensive income	(2,341)	(137)	(2,376)	(188)
Ending Balance	65,930	2,368	65,930	2,368
Unrealized Non-Credit Gains (Losses) on Other-Than-Temporarily Impaired Debt Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	0	598	0	680
Other comprehensive income before reclassifications	0	(600)	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	(14,746)	(14,798)	(15,001)	(15,089)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income	255	275	510	566
Ending Balance	(14,491)	(14,523)	(14,491)	(14,523)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	18,308	(37,633)	(137)	(59,063)
Other comprehensive income before reclassifications	34,424	24,317	51,853	44,533
Amounts reclassified from accumulated other comprehensive income	(2,086)	138	(1,866)	378
Ending Balance	51,439	(12,157)	51,439	(12,157)
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from accumulated other comprehensive income	793	1,021	1,589	1,995
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	793	1,021	1,589	1,995
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