Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income, Changes In (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ 858,597	$ 795,800	$ 875,297	$ 804,580
Income tax benefit (expense)		(6,726)	(2,518)	1,114	(10,307)
Ending balance, net of tax		894,400	817,605	894,400	817,605
Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		76,131	82,330	116,078	38,038
Other comprehensive (loss) income before reclassifications		33,166	25,813	(15,813)	87,299
Reclassification adjustments included in net income		375	(2,447)	(1,212)	(7,867)
Income tax benefit (expense)		(7,044)	(4,906)	3,575	(16,680)
Net current-period other comprehensive (loss) income, net of tax		26,497	18,460	(13,450)	62,752
Ending balance, net of tax		102,628	100,790	102,628	100,790
Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(7,144)	(17,236)	(17,091)	(1,672)
Other comprehensive (loss) income before reclassifications		(3,104)	(5,439)	7,919	(25,039)
Reclassification adjustments included in net income		1,599	797	3,167	696
Income tax benefit (expense)		315	974	(2,329)	5,111
Net current-period other comprehensive (loss) income, net of tax		(1,190)	(3,668)	8,757	(19,232)
Ending balance, net of tax		(8,334)	(20,904)	(8,334)	(20,904)
Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	0	(3)	0	(4)
Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	(9)	664	632	1,392
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		39,586	33,539	69,080	4,236
Other comprehensive (loss) income before reclassifications		30,062	12,979	(7,894)	54,865
Reclassification adjustments included in net income		1,965	(989)	2,587	(5,783)
Income tax benefit (expense)		(6,726)	(2,518)	1,114	(10,307)
Net current-period other comprehensive (loss) income, net of tax		25,301	9,472	(4,193)	38,775
Ending balance, net of tax		64,887	43,011	64,887	43,011
Pension Plans | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		149	(146)	149	(145)
Other comprehensive (loss) income before reclassifications		0	163	0	163
Reclassification adjustments included in net income		0	(3)	0	(4)
Income tax benefit (expense)		0	(34)	0	(34)
Net current-period other comprehensive (loss) income, net of tax		0	126	0	125
Ending balance, net of tax		149	(20)	149	(20)
Pension Plans | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(29,550)	(31,409)	(30,056)	(31,985)
Other comprehensive (loss) income before reclassifications		0	(7,558)	0	(7,558)
Reclassification adjustments included in net income		(9)	664	632	1,392
Income tax benefit (expense)		3	1,448	(132)	1,296
Net current-period other comprehensive (loss) income, net of tax		(6)	(5,446)	500	(4,870)
Ending balance, net of tax		$ (29,556)	$ (36,855)	$ (29,556)	$ (36,855)

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