Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,370,793	$ 2,054,998
Ending balance	2,435,707	2,159,471
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 (loss)	44,534	19,301
Ending balance	60,342	(9,491)
Accumulated Unrealized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	819	(2,550)
Other comprehensive income (loss)	(7,257)	1,052
Ending balance	(6,438)	(1,498)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(18,581)	(21,170)
Other comprehensive income (loss)	0	0
Ending balance	(18,581)	(21,170)
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,954)	(52,512)
Other comprehensive income (loss)	37,277	20,353
Ending balance	$ 35,323	$ (32,159)