Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 4,592,120	$ 4,384,188
Other comprehensive (loss) income before reclassification	(10,262)
Amounts reclassified from other comprehensive income	975
Total other comprehensive (loss) income	(9,287)	25,648
Ending balance	4,659,670	4,420,998
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(7,718)	(32,214)
Ending balance	(17,005)	$ (6,566)
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	33,290
Other comprehensive (loss) income before reclassification	(10,436)
Amounts reclassified from other comprehensive income	44
Total other comprehensive (loss) income	(10,392)
Ending balance	22,898
Unrealized Gains and Losses on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(3,906)
Other comprehensive (loss) income before reclassification	174
Amounts reclassified from other comprehensive income	651
Total other comprehensive (loss) income	825
Ending balance	(3,081)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(37,102)
Other comprehensive (loss) income before reclassification	0
Amounts reclassified from other comprehensive income	280
Total other comprehensive (loss) income	280
Ending balance	$ (36,822)