Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income, Changes In (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ 875,297	$ 804,580
Income tax benefit (expense)		7,840	(7,789)
Ending balance, net of tax		858,597	795,800
Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		116,078	38,038
Other comprehensive (loss) income before reclassifications		(48,979)	61,486
Reclassification adjustments included in net income		(1,587)	(5,420)
Income tax benefit (expense)		10,619	(11,774)
Net current-period other comprehensive (loss) income, net of tax		(39,947)	44,292
Ending balance, net of tax		76,131	82,330
Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(17,091)	(1,672)
Other comprehensive (loss) income before reclassifications		11,023	(19,600)
Reclassification adjustments included in net income		1,568	(101)
Income tax benefit (expense)		(2,644)	4,137
Net current-period other comprehensive (loss) income, net of tax		9,947	(15,564)
Ending balance, net of tax		(7,144)	(17,236)
Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	0	(1)
Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	641	728
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		69,080	4,236
Other comprehensive (loss) income before reclassifications		(37,956)	41,886
Reclassification adjustments included in net income		622	(4,794)
Income tax benefit (expense)		7,840	(7,789)
Net current-period other comprehensive (loss) income, net of tax		(29,494)	29,303
Ending balance, net of tax		39,586	33,539
Pension Plans | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		149	(145)
Other comprehensive (loss) income before reclassifications		0	0
Reclassification adjustments included in net income		0	(1)
Income tax benefit (expense)		0	0
Net current-period other comprehensive (loss) income, net of tax		0	(1)
Ending balance, net of tax		149	(146)
Pension Plans | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(30,056)	(31,985)
Other comprehensive (loss) income before reclassifications		0	0
Reclassification adjustments included in net income		641	728
Income tax benefit (expense)		(135)	(152)
Net current-period other comprehensive (loss) income, net of tax		506	576
Ending balance, net of tax		$ (29,550)	$ (31,409)

[1] These AOCI components are included in the computation of net periodic pension cost (income) presented in “Note 8 – Employee Benefit Plans.”