Accumulated Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	$ 2,692,958	$ 2,340,501	$ 2,616,828	$ 2,342,176
OCI before reclassifications	(21,622)	4,333	(16,073)	56,186
Amounts reclassified from AOCI	(601)	255	(25,557)	(1,612)
Ending Balance	2,699,818	2,390,265	2,699,818	2,390,265
Unrealized Gains (Losses) on Investment Securities
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	62,769	65,930	81,604	14,864
OCI before reclassifications	(21,622)	4,333	(16,073)	56,186
Amounts reclassified from AOCI	0	0	(25,901)	(2,377)
Ending Balance	41,784	71,190	41,784	71,190
Net Unrealized (Loss) Gain on Interest Rate Swaps used in Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	367	0	0	0
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCI	(891)	0	(524)	0
Ending Balance	(524)	0	(524)	0
Unrecognized Pension and Postretirement Plan Income (Costs)
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	(15,935)	(14,491)	(16,513)	(15,001)
OCI before reclassifications	0	0	0	0
Amounts reclassified from AOCI	290	255	868	765
Ending Balance	(15,645)	(14,236)	(15,645)	(14,236)
Total
Accumulated Other Comprehensive Income [Roll Forward]
Beginning Balance	47,201	51,439	65,091	(137)
Ending Balance	25,615	56,954	25,615	56,954
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from AOCI	637	928	2,154	2,517
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Gains (Losses) on Investment Securities
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from AOCI	637	928	2,154	2,517
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized (Loss) Gain on Interest Rate Swaps used in Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Amounts reclassified from AOCI	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 AOCI	$ 0	$ 0	$ 0	$ 0