Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,370,793	$ 2,054,998
Ending balance	2,664,171	2,339,599
Accumulated Unrealized (Loss) Gain on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	15,808	(28,792)
Other comprehensive income	29,876	56,725
Ending balance	45,684	27,933
Accumulated Unrealized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	819	(2,550)
Other comprehensive income	(2,173)	2,525
Ending balance	(1,354)	(25)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(18,581)	(21,170)
Other comprehensive income	0	0
Ending balance	(18,581)	(21,170)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,954)	(52,512)
Other comprehensive income	27,703	59,250
Ending balance	$ 25,749	$ 6,738