Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net of tax
Balance, beginning	$ 2,659,441	$ 2,459,175	$ 2,524,839	$ 2,532,551
Net income	74,199	67,388	216,556	204,399
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
ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(20,068)
Accumulated other comprehensive Income (loss)
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) | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(20,068)
Pension and other benefits, net change in pension and other benefits
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 | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(5,393)
Investment securities:
Net of tax
Balance, beginning	(3,011)	(156,373)	(103,816)	(74,117)
Other comprehensive income (loss)	13,210	(22,420)	114,015	(89,236)
Balance, ending	$ 10,199	(178,793)	$ 10,199	(178,793)
Investment securities: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				(15,440)
Cash flow derivative hedges:
Net of tax
Balance, beginning		5,302		3,680
Other comprehensive income (loss)		131		988
Balance, ending		$ 5,433		5,433
Cash flow derivative hedges: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)				$ 765