Accumulated Other Comprehensive Income (Loss) - Accumulated Other Comprehensive Income, Changes In (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		$ 817,605	$ 787,765	$ 804,580	$ 731,291
Income tax (expense) benefit		(866)	(3,125)	(11,173)	(17,789)
Ending balance, net of tax		839,148	810,453	839,148	810,453
Unrealized Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		100,790	31,070	38,038	(31,120)
Other comprehensive income (loss) before reclassifications		1,601	17,334	88,900	96,156
Reclassification adjustments included in net income		316	70	(7,551)	(31)
Income tax (expense) benefit		(403)	(3,655)	(17,083)	(20,186)
Net current-period other comprehensive income (loss), net of tax		1,514	13,749	64,266	75,939
Ending balance, net of tax		102,304	44,819	102,304	44,819
Unrealized Gains (Losses) on Derivatives
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(20,904)	(820)	(1,672)	7,146
Other comprehensive income (loss) before reclassifications		(197)	(2,619)	(25,236)	(11,392)
Reclassification adjustments included in net income		1,587	(499)	2,283	(1,809)
Income tax (expense) benefit		(292)	655	4,819	2,772
Net current-period other comprehensive income (loss), net of tax		1,098	(2,463)	(18,134)	(10,429)
Ending balance, net of tax		(19,806)	(3,283)	(19,806)	(3,283)
Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	(2)	(1)	(6)	(5)
Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Reclassification adjustments included in net income	[1]	821	596	2,213	1,789
Total
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		43,011	4,935	4,236	(50,228)
Other comprehensive income (loss) before reclassifications		1,404	14,715	56,269	84,764
Reclassification adjustments included in net income		2,722	166	(3,061)	(56)
Income tax (expense) benefit		(866)	(3,125)	(11,173)	(17,789)
Net current-period other comprehensive income (loss), net of tax		3,260	11,756	42,035	66,919
Ending balance, net of tax		46,271	16,691	46,271	16,691
Defined Benefit Pension Plan | Net Prior Service (Cost) Credit
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(20)	(142)	(145)	(139)
Other comprehensive income (loss) before reclassifications		0	0	163	0
Reclassification adjustments included in net income		(2)	(1)	(6)	(5)
Income tax (expense) benefit		0	0	(34)	1
Net current-period other comprehensive income (loss), net of tax		(2)	(1)	123	(4)
Ending balance, net of tax		(22)	(143)	(22)	(143)
Defined Benefit Pension Plan | Net Gain (Loss)
AOCI Attributable to Parent, Net of Tax
Beginning balance, net of tax		(36,855)	(25,173)	(31,985)	(26,115)
Other comprehensive income (loss) before reclassifications		0	0	(7,558)	0
Reclassification adjustments included in net income		821	596	2,213	1,789
Income tax (expense) benefit		(171)	(125)	1,125	(376)
Net current-period other comprehensive income (loss), net of tax		650	471	(4,220)	1,413
Ending balance, net of tax		$ (36,205)	$ (24,702)	$ (36,205)	$ (24,702)

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