Accumulated Other Comprehensive Income (Loss) - Changes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pre-tax Amount
Balance	$ (180,915)	$ (159,423)	$ (145,472)
Other comprehensive income (loss)	137,465	(21,492)	(13,951)
Balance	(43,450)	(180,915)	(159,423)
Income Tax Benefit (Expense)
Balance	48,720	63,040	57,461
Other comprehensive income (loss)	(37,019)	5,748	5,579
Balance	11,701	48,720	63,040
Net of tax
Balance, beginning	2,524,839	2,532,551	2,476,485
Other comprehensive income (loss)	100,446	(15,744)	(8,372)
Balance, ending	2,640,258	2,524,839	2,532,551
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	(132,195)	(96,383)	(88,011)
Other comprehensive income (loss)	100,446	(15,744)	(8,372)
Balance, ending	(31,749)	(132,195)	(96,383)
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
Other comprehensive income (loss)	(20,068)
Pension and other benefits, net actuarial losses arising during the year
Pre-tax Amount
Unrealized net (losses) gains arising during the year	(5,774)	(2,835)	(1,104)
Reclassification of net losses (gains) to net income	6,610	7,315	8,625
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the year	1,555	763	436
Reclassification of net losses (gains) to net income	(1,780)	(1,970)	(3,407)
Net of tax
Unrealized net gains (losses) arising during the year	(4,219)	(2,072)	(668)
Reclassification of net losses (gains) to net income	4,830	5,345	5,218
Pension and other benefits, prior service credit
Pre-tax Amount
Unrealized net (losses) gains arising during the year	(429)
Reclassification of net losses (gains) to net income		(429)	(429)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the year	115
Reclassification of net losses (gains) to net income		116	170
Net of tax
Unrealized net gains (losses) arising during the year	(314)
Reclassification of net losses (gains) to net income		(313)	(259)
Pension and other benefits, net change in pension and other benefits
Pre-tax Amount
Other comprehensive income (loss)	407	4,051	7,092
Income Tax Benefit (Expense)
Other comprehensive income (loss)	(110)	(1,091)	(2,801)
Net of tax
Balance, beginning	(28,379)	(25,946)	(30,237)
Other comprehensive income (loss)	297	2,960	4,291
Balance, ending	(28,082)	(28,379)	(25,946)
Pension and other benefits, net change in pension and other benefits | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)	(5,393)
Investment securities:
Pre-tax Amount
Unrealized net (losses) gains arising during the year	134,343	(43,545)	(23,516)
Other comprehensive income (loss)	137,058	(19,460)	(23,516)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the year	(36,178)	11,686	9,357
Other comprehensive income (loss)	(36,909)	5,201	9,357
Net of tax
Balance, beginning	(103,816)	(74,117)	(59,958)
Unrealized net gains (losses) arising during the year	98,165	(31,859)	(14,159)
Other comprehensive income (loss)	100,149	(14,259)	(14,159)
Balance, ending	(3,667)	(103,816)	(74,117)
Investment securities: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)	(15,440)
Investment securities, OTTI losses on AFS debt securities, net
Pre-tax Amount
Reclassification of net losses (gains) to net income	2,715	24,085
Income Tax Benefit (Expense)
Reclassification of net losses (gains) to net income	731	(6,485)
Net of tax
Reclassification of net losses (gains) to net income	1,984	17,600
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net (losses) gains arising during the year		1,475	2,473
Reclassification of net losses (gains) to net income		(7,558)
Other comprehensive income (loss)		(6,083)	2,473
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the year		(397)	(977)
Reclassification of net losses (gains) to net income		(2,035)
Other comprehensive income (loss)		1,638	(977)
Net of tax
Balance, beginning		3,680	2,184
Unrealized net gains (losses) arising during the year		1,078	1,496
Reclassification of net losses (gains) to net income		(5,523)
Other comprehensive income (loss)		$ (4,445)	1,496
Balance, ending			$ 3,680
Cash flow derivative hedges: | Early Adoption | ASU 2018-02
Net of tax
Other comprehensive income (loss)	$ 765