Accumulated Other Comprehensive Loss - Changes (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pre-tax Amount
Other comprehensive income (loss)	$ 18,079	$ (30,504)	$ 155,222	$ (120,729)
Income Tax Benefit (Expense)
Other comprehensive income (loss)	(4,869)	8,215	(41,801)	32,481
Net of tax
Balance, beginning	2,659,441	2,459,175	2,524,839	2,532,551
Other comprehensive income (loss)	13,210	(22,289)	113,421	(88,248)
Balance, ending	2,654,558	2,423,462	2,654,558	2,423,462
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)
Accumulated other comprehensive Income (loss)
Pre-tax Amount
Balance	(43,772)	(249,648)	(180,915)	(159,423)
Balance	(25,693)	(280,152)	(25,693)	(280,152)
Income Tax Benefit (Expense)
Balance	11,788	67,238	48,720	63,040
Balance	6,919	75,453	6,919	75,453
Net of tax
Balance, beginning	(31,984)	(182,410)	(132,195)	(96,383)
Other comprehensive income (loss)	13,210	(22,289)	113,421	(88,248)
Balance, ending	(18,774)	(204,699)	(18,774)	(204,699)
Accumulated other comprehensive Income (loss) \| Early Adoption \| ASU 2018-02
Net of tax
Other comprehensive income (loss)				(20,068)
Pension and other benefits, net actuarial losses arising during the year
Pre-tax Amount
Other comprehensive income (loss)			(813)
Income Tax Benefit (Expense)
Other comprehensive income (loss)			219
Net of tax
Other comprehensive income (loss)			(594)
Pension and other benefits, net change in pension and other benefits
Pre-tax Amount
Other comprehensive income (loss)			(813)
Income Tax Benefit (Expense)
Other comprehensive income (loss)			219
Net of tax
Balance, beginning	(28,973)	(31,339)	(28,379)	(25,946)
Other comprehensive income (loss)			(594)
Balance, ending	(28,973)	(31,339)	(28,973)	(31,339)
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 period	18,079	(30,682)	153,443	(122,070)
Reclassification of net losses (gains) to net income			2,592
Other comprehensive income (loss)	18,079	(30,682)	156,035	(122,070)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(4,869)	8,262	(41,322)	32,834
Reclassification of net losses (gains) to net income			(698)
Other comprehensive income (loss)	(4,869)	8,262	(42,020)	32,834
Net of tax
Balance, beginning	(3,011)	(156,373)	(103,816)	(74,117)
Unrealized net gains (losses) arising during the period	13,210	(22,420)	112,121	(89,236)
Reclassification of net losses (gains) to net income			1,894
Other comprehensive income (loss)	13,210	(22,420)	114,015	(89,236)
Balance, ending	$ 10,199	(178,793)	$ 10,199	(178,793)
Investment securities: \| Early Adoption \| ASU 2018-02
Net of tax
Other comprehensive income (loss)				(15,440)
Cash flow derivative hedges:
Pre-tax Amount
Unrealized net (losses) gains arising during the period		178		1,341
Other comprehensive income (loss)		178		1,341
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period		(47)		(353)
Other comprehensive income (loss)		(47)		(353)
Net of tax
Balance, beginning		5,302		3,680
Unrealized net gains (losses) arising during the period		131		988
Other comprehensive income (loss)		131		988
Balance, ending		$ 5,433		5,433
Cash flow derivative hedges: \| Early Adoption \| ASU 2018-02
Net of tax
Other comprehensive income (loss)				$ 765

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