Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income, Changes In (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ 894,400	$ 817,605	$ 875,297	$ 804,580
Income tax benefit (expense)		5,560	(866)	6,674	(11,173)
Ending balance, net of tax		877,866	839,148	877,866	839,148
Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		102,628	100,790	116,078	38,038
Other comprehensive (loss) income before reclassifications		(27,714)	1,601	(43,527)	88,900
Reclassification adjustments included in net income		(1,069)	316	(2,281)	(7,551)
Income tax benefit (expense)		6,045	(403)	9,620	(17,083)
Net current-period other comprehensive (loss) income, net of tax		(22,738)	1,514	(36,188)	64,266
Ending balance, net of tax		79,890	102,304	79,890	102,304
Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(8,334)	(20,904)	(17,091)	(1,672)
Other comprehensive (loss) income before reclassifications		377	(197)	8,296	(25,236)
Reclassification adjustments included in net income		1,617	1,587	4,784	2,283
Income tax benefit (expense)		(418)	(292)	(2,747)	4,819
Net current-period other comprehensive (loss) income, net of tax		1,576	1,098	10,333	(18,134)
Ending balance, net of tax		(6,758)	(19,806)	(6,758)	(19,806)
Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	0	(2)	0	(6)
Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	316	821	948	2,213
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		64,887	43,011	69,080	4,236
Other comprehensive (loss) income before reclassifications		(27,337)	1,404	(35,231)	56,269
Reclassification adjustments included in net income		864	2,722	3,451	(3,061)
Income tax benefit (expense)		5,560	(866)	6,674	(11,173)
Net current-period other comprehensive (loss) income, net of tax		(20,913)	3,260	(25,106)	42,035
Ending balance, net of tax		43,974	46,271	43,974	46,271
Pension Plans | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		149	(20)	149	(145)
Other comprehensive (loss) income before reclassifications		0	0	0	163
Reclassification adjustments included in net income		0	(2)	0	(6)
Income tax benefit (expense)		0	0	0	(34)
Net current-period other comprehensive (loss) income, net of tax		0	(2)	0	123
Ending balance, net of tax		149	(22)	149	(22)
Pension Plans | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(29,556)	(36,855)	(30,056)	(31,985)
Other comprehensive (loss) income before reclassifications		0	0	0	(7,558)
Reclassification adjustments included in net income		316	821	948	2,213
Income tax benefit (expense)		(67)	(171)	(199)	1,125
Net current-period other comprehensive (loss) income, net of tax		249	650	749	(4,220)
Ending balance, net of tax		$ (29,307)	$ (36,205)	$ (29,307)	$ (36,205)

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