Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 2,690,006	$ 2,370,793
Ending balance	2,723,223	2,664,171
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)	(47,288)	29,876
Ending balance	(8,960)	45,684
Accumulated Unrealized Gain (Loss) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	10,179	819
Other comprehensive income (loss)	(4,472)	(2,173)
Ending balance	5,707	(1,354)
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)	(51,760)	27,703
Ending balance	$ (25,381)	$ 25,749