Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		$ 2,370,793		$ 2,054,998		$ 1,864,874
Other comprehensive (loss) income		28,333		50,558		(12,787)
Ending balance		2,690,006		2,370,793		2,054,998
Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(26,821)	[1]	47,257	[2]	0
Ending balance				(26,821)	[1]	47,257	[2]
Accumulated Unrealized Gain (Loss) on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		15,808		(28,792)		(13,976)
Other comprehensive (loss) income		22,520		44,600		(11,952)
Ending balance		38,328		15,808		(28,792)
Accumulated Unrealized Gain (Loss) on Securities Available- for-Sale | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	[3]					(2,864)
Accumulated Unrealized Gain (Loss) on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		819		(2,550)		(3,763)
Other comprehensive (loss) income		9,360		3,369		1,997
Ending balance		10,179		819		(2,550)
Accumulated Unrealized Gain (Loss) on Derivative Instruments | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	[3]					(784)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(18,581)		(21,170)		(15,297)
Other comprehensive (loss) income		(3,547)		2,589		(2,832)
Ending balance		(22,128)		(18,581)		(21,170)
Unrecognized Net Pension Costs | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	[3]					(3,041)
Total Accumulated Other Comprehensive Income (Loss), Net of Tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(1,954)		(52,512)		(33,036)
Other comprehensive (loss) income		28,333		50,558		(12,787)
Ending balance		$ 26,379		$ (1,954)		(52,512)
Total Accumulated Other Comprehensive Income (Loss), Net of Tax | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	[3]					$ (6,689)

[1] As a result of accounting guidance adopted in the first quarter of 2020, a portion of the increase in allowance for credit losses, net of tax, was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2020. For further discussion of this guidance, see Note 2, "Recent Accounting Pronouncements."

[2] As a result of accounting guidance adopted in 2019, the remaining deferred gain on a sale-leaseback transaction was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2019.

[3] As a result of accounting guidance adopted in 2018, certain reclassifications were made from accumulated other comprehensive loss to retained earnings as of January 1, 2018.