Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,690,006	$ 2,370,793
Ending balance	2,704,148	2,656,211
Accumulated Unrealized Gain (Loss) on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	38,328	15,808
Other comprehensive income (loss)	(29,362)	43,196
Ending balance	8,966	59,004
Accumulated Unrealized Gain (Loss) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	10,179	819
Other comprehensive income (loss)	(2,958)	(12,515)
Ending balance	7,221	(11,696)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(22,128)	(18,581)
Other comprehensive income (loss)	0	0
Ending balance	(22,128)	(18,581)
Total Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	26,379	(1,954)
Other comprehensive income (loss)	(32,320)	30,681
Ending balance	$ (5,941)	$ 28,727