Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income		$ 40,007	$ 34,873	$ 119,290	$ 86,526
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Securities, Available-for-sale, Adjustment, after Tax		(7,897)	(709)	(11,878)	10,333
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax		(3,383)	(2,729)	(11,559)	20,452
Net change in other comprehensive income for defined benefit postretirement plans	[1]	280	225	1,309	(322)
Total other comprehensive income (loss)		(11,000)	(3,213)	(22,128)	30,463
Total comprehensive income		$ 29,007	$ 31,660	$ 97,162	$ 116,989

[1] The following tables present a reconciliation of the changes in the components of other comprehensive income (loss) for the periods indicated, including the amount of income tax (expense) benefit allocated to each component of other comprehensive income (loss): Three Months Ended
September 30, 2021Nine Months Ended
September 30, 2021Pre Tax
AmountTax (Expense)
BenefitAfter Tax
AmountPre Tax
AmountTax (Expense)
BenefitAfter Tax
Amount (Dollars in thousands)Change in fair value of securities available for sale$(10,337)$2,440 $(7,897)$(15,589)$3,711 $(11,878)Less: net security losses reclassified into other noninterest expense— — — — — — Net change in fair value of securities available for sale(10,337)2,440 (7,897)(15,589)3,711 (11,878)Change in fair value of cash flow hedges84 (23)61 (2,214)624 (1,590)Less: net cash flow hedge gains reclassified into interest income or interest expense 4,791 (1,347)3,444 13,869 (3,900)9,969 Net change in fair value of cash flow hedges(4,707)1,324 (3,383)(16,083)4,524 (11,559)Net unamortized gain related to defined benefit pension and other postretirement adjustments arising during the period— — — 653 (184)469 Amortization of net actuarial losses346 (97)249 1,037 (291)746 Amortization of net prior service costs44 (13)31 131 (37)94 Net change in other comprehensive income for defined benefit postretirement plans (1)390 (110)280 1,821 (512)1,309 Total other comprehensive loss$(14,654)$3,654 $(11,000)$(29,851)$7,723 $(22,128)  Three Months Ended
September 30, 2020Nine Months Ended
September 30, 2020 Pre Tax
AmountTax (Expense)
BenefitAfter Tax
AmountPre Tax
AmountTax (Expense)
BenefitAfter Tax
Amount (Dollars in thousands)Change in fair value of securities available for sale$(857)$148 $(709)$13,598 $(3,265)$10,333 Less: net security losses reclassified into other noninterest expense— — — — — — Net change in fair value of securities available for sale(857)148 (709)13,598 (3,265)10,333 Change in fair value of cash flow hedges(143)41 (102)37,674 (10,597)27,077 Less: net cash flow hedge gains reclassified into interest income or interest expense 4,339 (1,220)3,119 9,901 (2,784)7,117 Less: loss on termination of hedge reclassified into noninterest expense(684)192 (492)(684)192 (492)Net change in fair value of cash flow hedges(3,798)1,069 (2,729)28,457 (8,005)20,452 Net unamortized loss related to defined benefit pension and other postretirement adjustments arising during the period(2)1 (1)(1,392)392 (1,000)Amortization of net actuarial losses245 (69)176 736 (207)529 Amortization of net prior service costs69 (19)50 207 (58)149 Net change in other comprehensive income for defined benefit postretirement plans (1)312 (87)225 (449)127 (322)Total other comprehensive income (loss)$(4,343)$1,130 $(3,213)$41,606 $(11,143)$30,463 
(1)The amortization of prior service costs is included in the computation of net periodic pension cost as disclosed in Note 15 "Employee Benefit Plans" within the Notes to the Consolidated Financial Statements included in Item 8 of the Company's 2020 Form 10-K.