Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net of tax
Balance			$ 2,532,551	$ 2,476,485
Other comprehensive income (loss)	$ (17,726)	$ 19,041	(65,959)	20,360
Balance	2,459,175	2,552,602	2,459,175	2,552,602
Accumulated other comprehensive loss
Net of tax
Balance	(164,684)	(86,692)	(96,383)	(88,011)
Other comprehensive income (loss)	(17,726)	19,041	(65,959)	20,360
Balance	(182,410)	(67,651)	(182,410)	(67,651)
Accumulated other comprehensive loss | ASU 2018-02
Net of tax
Adoption of ASU	(20,068)		(20,068)
Accumulated other comprehensive loss | ASU 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Net of tax
Adoption of ASU	(20,068)		(20,068)
Pension and other benefits:
Net of tax
Balance	(31,339)	(30,237)	(25,946)	(30,237)
Balance	(31,339)	(30,237)	(31,339)	(30,237)
Pension and other benefits: | ASU 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Net of tax
Adoption of ASU	(5,393)		(5,393)
Investment securities:
Net of tax
Balance	(138,334)	(59,001)	(74,117)	(59,958)
Other comprehensive income (loss)	(18,039)	18,885	(66,816)	19,842
Balance	(156,373)	(40,116)	(156,373)	(40,116)
Investment securities: | ASU 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Net of tax
Adoption of ASU	(15,440)		(15,440)
Cash flow derivative hedges:
Net of tax
Balance	4,989	2,546	3,680	2,184
Other comprehensive income (loss)	313	156	857	518
Balance	5,302	$ 2,702	5,302	$ 2,702
Cash flow derivative hedges: | ASU 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Net of tax
Adoption of ASU	$ 765		$ 765