Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive 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 (Loss)
Beginning balance	$ 4,737,807	$ 4,474,488	$ 4,592,120	$ 4,384,188
Other comprehensive loss before reclassification	(4,049)		(15,929)
Amounts reclassified from other comprehensive (loss) income	409		2,272
Total other comprehensive (loss) income	(3,640)	155	(13,657)	27,431
Ending balance	4,822,498	4,533,763	4,822,498	4,533,763
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(17,735)	(4,938)	(7,718)	(32,214)
Ending balance	(21,375)	$ (4,783)	(21,375)	$ (4,783)
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	21,503		33,290
Other comprehensive loss before reclassification	(3,953)		(15,860)
Amounts reclassified from other comprehensive (loss) income	(613)		(493)
Total other comprehensive (loss) income	(4,566)		(16,353)
Ending balance	16,937		16,937
Unrealized Gains and Losses on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(2,694)		(3,906)
Other comprehensive loss before reclassification	(96)		(69)
Amounts reclassified from other comprehensive (loss) income	743		1,928
Total other comprehensive (loss) income	647		1,859
Ending balance	(2,047)		(2,047)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(36,544)		(37,102)
Other comprehensive loss before reclassification	0		0
Amounts reclassified from other comprehensive (loss) income	279		837
Total other comprehensive (loss) income	279		837
Ending balance	$ (36,265)		$ (36,265)