Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net of tax
Balance, beginning	$ 2,524,839	$ 2,532,551
Net income	69,924	67,958
Other comprehensive income (loss)	53,441	(48,233)
Balance, ending	2,613,202	2,520,862
ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)		(20,068)
Accumulated other comprehensive Income (loss)
Net of tax
Balance, beginning	(132,195)	(96,383)
Other comprehensive income (loss)	53,441	(48,233)
Balance, ending	(78,754)	(164,684)
Accumulated other comprehensive Income (loss) | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)		(20,068)
Pension and other benefits:
Net of tax
Balance, beginning	(28,379)	(25,946)
Balance, ending	(28,379)	(31,339)
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	(103,816)	(74,117)
Other comprehensive income (loss)	53,441	(48,777)
Balance, ending	$ (50,375)	(138,334)
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		3,680
Other comprehensive income (loss)		544
Balance, ending		4,989
Cash flow derivative hedges: | ASU 2018-02 | Early Adoption
Net of tax
Other comprehensive income (loss)		$ 765