Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 40,691	$ 57,218
Net income, Tax (Expense) Benefit	(8,807)	(12,956)
Net income, Net of Tax Amount	31,884	44,262
Unrealized gains on available-for-sale securities:
Unrealized holding gains arising during period, Before-tax Amount	13,685	33,174
Unrealized holding gains arising during period, Tax (Expense) Benefit	(3,433)	(8,049)
Unrealized holding gains arising during period, Net of Tax Amount	10,252	25,125
Reclassification adjustment for losses included in net income, Before-tax Amount		267
Reclassification adjustment for losses included in net income, Tax (Expense) Benefit	0	(68)
Reclassification adjustment for losses included in net income, Net of Tax Amount		199
Net unrealized gains, Before-tax Amount		33,441
Net unrealized gains, Tax (Expense) Benefit		(8,117)
Net unrealized gains, Net of Tax Amount		25,324
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Before-tax Amount	83	84
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Tax (Expense) Benefit	(20)	(20)
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Net of Tax Amount	63	64
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Before-tax Amount		102
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Tax (Expense) Benefit		(26)
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, After-tax Amount		76
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Before-tax Amount	214	174
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Tax (Expense) Benefit	(54)	(44)
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Net of Tax Amount	160	130
Total other comprehensive income, Before-tax Amount	13,982	33,801
Total other comprehensive income, Tax (Expense) Benefit	(3,507)	(8,207)
Total other comprehensive income, net of tax amount	10,475	25,594
Comprehensive income, Before-tax Amount	54,673	91,019
Comprehensive income, Tax (Expense) Benefit	(12,314)	(21,163)
Comprehensive income, Net of Tax Amount	$ 42,359	$ 69,856