Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pre-tax Amount
Other comprehensive income (loss), Pre-tax Amount	$ 86,548	$ 137,465	$ (21,492)
Unrealized net gains on cash flow derivative hedges arising during the year, Pre-tax Amount			(6,083)
Income Tax Benefit (Expense)
Other comprehensive income (loss), Income Tax Benefit (Expense)	(23,195)	(37,019)	5,748
Unrealized net gains on cash flow derivative hedges arising during the year, Income Tax Benefit (Expense)			1,638
Net of tax
Balance	2,640,258	2,524,839	2,532,551
Other comprehensive income (loss), Net of tax	63,353	100,446	(15,744)
Unrealized net gains on cash flow derivative hedges arising during the year, Net of tax			(4,445)
Balance	2,744,104	2,640,258	2,524,839
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Accumulated other comprehensive loss, beginning balance, Pre-tax Amount	(43,450)	(180,915)	(159,423)
Accumulated other comprehensive loss, ending balance, Pre-tax Amount	43,098	(43,450)	(180,915)
Income Tax Benefit (Expense)
Accumulated other comprehensive loss, beginning balance, Income Tax Benefit (Expense)	11,701	48,720	63,040
Accumulated other comprehensive loss, ending balance, Income Tax Benefit (Expense)	(11,494)	11,701	48,720
Net of tax
Balance	(31,749)	(132,195)	(96,383)
Other comprehensive income (loss), Net of tax	63,353	100,446	(15,744)
Balance	31,604	(31,749)	(132,195)
Accumulated other comprehensive Income (loss) | Early Adoption | ASU 2018-02
Income Tax Benefit (Expense)
Early adoption of ASU No. 2018-02			(20,068)
Net of tax
Balance		(20,068)
Other comprehensive income (loss), Net of tax			(20,068)
Early adoption of ASU No. 2018-02			(20,068)
Balance			(20,068)
Pension and other benefits:
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount	(4,855)	407	4,051
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	1,200	(110)	(1,091)
Net of tax
Balance	(28,082)	(28,379)	(25,946)
Unrealized net gains arising during the period, Net of tax	(3,655)	297	2,960
Other comprehensive income (loss), Net of tax	(3,655)	297	2,960
Balance	(31,737)	(28,082)	(28,379)
Pension and other benefits: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss), Net of tax			(5,393)
Pension and other benefits, net actuarial losses arising during the period
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount	(10,399)	(5,774)	(2,835)
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	2,774	1,555	763
Net of tax
Unrealized net gains arising during the period, Net of tax	(7,625)	(4,219)	(2,072)
Pension and other benefits, prior service credit
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount	(51)	(429)	(429)
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	14	115	116
Net of tax
Unrealized net gains arising during the period, Net of tax	(37)	(314)	(313)
Pension and other benefits: Amortization of net loss included in net income
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount	5,595	6,610	7,315
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	(1,492)	(1,780)	(1,970)
Net of tax
Unrealized net gains arising during the period, Net of tax	4,103	4,830	5,345
Pension and other benefits, change in Company tax rate
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	(96)
Net of tax
Unrealized net gains arising during the period, Net of tax	(96)
Investment securities:
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount			(19,460)
Reclassification of net losses (gains) to net income, Pre-tax Amount		2,715
Other comprehensive income (loss), Pre-tax Amount	91,403	137,058
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)			5,201
Reclassification of net losses (gains) to net income, Income Tax Benefit (Expense)		731
Other comprehensive income (loss), Income Tax Benefit (Expense)	(24,395)	(36,909)
Net of tax
Balance	(3,667)	(103,816)	(74,117)
Unrealized net gains arising during the period, Net of tax			(14,259)
Reclassification of net gains to net income, Net of tax		1,984
Other comprehensive income (loss), Net of tax	67,008	100,149	(14,259)
Balance	63,341	(3,667)	(103,816)
Investment securities: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss), Net of tax			(15,440)
Investment securities: Unrealized net gains arising during the year
Pre-tax Amount
Unrealized net gains (losses) arising during the year, Pre-tax Amount	91,289	134,343	(43,545)
Income Tax Benefit (Expense)
Unrealized net gains arising during the period, Income Tax Benefit (Expense)	(24,365)	(36,178)	11,686
Net of tax
Unrealized net gains arising during the period, Net of tax	66,924	$ 98,165	(31,859)
Investment securities: Investment securities losses, net
Pre-tax Amount
Reclassification of net losses (gains) to net income, Pre-tax Amount	114
Income Tax Benefit (Expense)
Reclassification of net losses (gains) to net income, Income Tax Benefit (Expense)	30
Net of tax
Reclassification of net gains to net income, Net of tax	$ 84
OTTI losses on available-for-sale debt securities, net
Pre-tax Amount
Reclassification of net losses (gains) to net income, Pre-tax Amount			24,085
Income Tax Benefit (Expense)
Reclassification of net losses (gains) to net income, Income Tax Benefit (Expense)			6,485
Net of tax
Reclassification of net gains to net income, Net of tax			17,600
Cash flow derivative hedges
Net of tax
Balance			3,680
Other comprehensive income (loss), Net of tax			(4,445)
Cash flow derivative hedges | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss), Net of tax			765
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net gains on cash flow derivative hedges arising during the year, Pre-tax Amount			1,475
Reclassification of net gains to net income: Other noninterest expense, Pre-tax Amount			(7,558)
Income Tax Benefit (Expense)
Unrealized net gains on cash flow derivative hedges arising during the year, Income Tax Benefit (Expense)			(397)
Reclassification of net gains to net income: Other noninterest expense, Income Tax Benefit (Expense)			2,035
Net of tax
Unrealized net gains on cash flow derivative hedges arising during the year, Net of tax			1,078
Reclassification of net gains to net income: Other noninterest expense, Net of tax			$ (5,523)