Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,370,793	$ 2,054,998
Ending balance	2,656,211	2,300,573
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)	43,196	46,889
Ending balance	59,004	18,097
Accumulated Unrealized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	819	(2,550)
Other comprehensive income (loss)	(12,515)	2,813
Ending balance	(11,696)	263
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)	30,681	49,702
Ending balance	$ 28,727	$ (2,810)