Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 59,362	$ 62,345	$ 132,072	$ 176,815
Net income, Tax (Expense) Benefit	(11,755)	(13,983)	(27,485)	(40,106)
Net income, Net of Tax Amount	47,607	48,362	104,587	136,709
Unrealized gains on available-for-sale securities:
Unrealized holding gains arising during period, Before-tax Amount	941	8,014	43,611	70,944
Unrealized holding gains arising during period, Tax (Expense) Benefit	(181)	(1,897)	(10,583)	(17,194)
Unrealized holding gains arising during period, Net of Tax Amount	760	6,117	33,028	53,750
Reclassification adjustment for losses included in net income, Before-tax Amount	(746)	0	(746)	118
Reclassification adjustment for losses included in net income, Tax (Expense) Benefit	191	0	191	(30)
Reclassification adjustment for losses included in net income, Net of Tax Amount	(555)	0	(555)	88
Net unrealized gains, Before-tax Amount	195	8,014	42,865	71,062
Net unrealized gains, Tax (Expense) Benefit	10	(1,897)	(10,392)	(17,224)
Net unrealized gains, Net of Tax Amount	205	6,117	32,473	53,838
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Before-tax Amount	544	105	723	282
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Tax (Expense) Benefit	(130)	(25)	(173)	(67)
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Net of Tax Amount	414	80	550	215
Derivative instruments designated as cash flow hedges:
Unrealized holding losses on derivatives arising during the period, Before-tax Amount	(324)		(1,152)
Unrealized holding losses on derivatives arising during the period, Tax (Expense) Benefit	83		294
Unrealized holding losses on derivatives arising during the period, Net of Tax Amount	(241)		(858)
Reclassification of losses on derivative instruments realized in net income, Before-tax Amount	130		197
Reclassification of losses on derivative instruments realized in net income, Tax (Expense) Benefit	(33)		(50)
Reclassification of losses on derivative instruments realized in net income, Net of Tax Amount	97		147
Net cash flow hedge activity, Before-tax Amount	(194)		(955)
Net cash flow hedge activity, Tax (Expense) Benefit	50		244
Net cash flow hedge activity, Net of tax Amount	(144)		(711)
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Before-tax Amount		0		337
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Tax (Expense) Benefit		0		(86)
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Net of Tax Amount		0		251
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax		1,160		1,160
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, Tax		(398)		(398)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax		1,558		1,558
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Before-tax Amount	215	174	643	521
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Tax (Expense) Benefit	(55)	(45)	(164)	(133)
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Net of Tax Amount	160	129	479	388
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax and Reclassification Adjustment, Attributable to Parent		1,289		1,548
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax		(443)		(531)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent		1,732		2,079
Total other comprehensive income, Before-tax Amount	760	9,851	43,276	73,760
Total other comprehensive income, Tax (Expense) Benefit	(125)	(2,365)	(10,485)	(17,908)
Total other comprehensive income, net of tax amount	635	7,486	32,791	55,852
Comprehensive income, Before-tax Amount	60,122	72,196	175,348	250,575
Comprehensive income, Tax (Expense) Benefit	(11,880)	(16,348)	(37,970)	(58,014)
Comprehensive income, Net of Tax Amount	$ 48,242	$ 55,848	$ 137,378	$ 192,561