Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 4,659,670	$ 4,420,998	$ 4,592,120	$ 4,384,188
Other comprehensive (loss) income before reclassification	(1,618)		(11,880)
Amounts reclassified from other comprehensive income	888		1,863
Total other comprehensive (loss) income	(730)	1,628	(10,017)	27,276
Ending balance	4,737,807	4,474,488	4,737,807	4,474,488
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(17,005)	(6,566)	(7,718)	(32,214)
Ending balance	(17,735)	$ (4,938)	(17,735)	$ (4,938)
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	22,898		33,290
Other comprehensive (loss) income before reclassification	(1,471)		(11,907)
Amounts reclassified from other comprehensive income	76		120
Total other comprehensive (loss) income	(1,395)		(11,787)
Ending balance	21,503		21,503
Unrealized Gains and Losses on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(3,081)		(3,906)
Other comprehensive (loss) income before reclassification	(147)		27
Amounts reclassified from other comprehensive income	534		1,185
Total other comprehensive (loss) income	387		1,212
Ending balance	(2,694)		(2,694)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(36,822)		(37,102)
Other comprehensive (loss) income before reclassification	0		0
Amounts reclassified from other comprehensive income	278		558
Total other comprehensive (loss) income	278		558
Ending balance	$ (36,544)		$ (36,544)