Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net of tax
Balance			$ 2,532,551	$ 2,476,485
Net income	$ 67,388	$ 58,363	204,399	171,998
Other comprehensive income (loss)	(22,289)	537	(88,248)	20,897
Balance	2,423,462	2,581,858	2,423,462	2,581,858
Accumulated other comprehensive loss
Net of tax
Balance	(182,410)	(67,651)	(96,383)	(88,011)
Other comprehensive income (loss)	(22,289)	537	(88,248)	20,897
Balance	(204,699)	(67,114)	(204,699)	(67,114)
Accumulated other comprehensive loss | ASU 2018-02 | Early Adoption
Net of tax
Net income			(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 | Early Adoption
Net of tax
Net income			(5,393)
Investment securities:
Net of tax
Balance	(156,373)	(40,116)	(74,117)	(59,958)
Other comprehensive income (loss)	(22,420)	128	(89,236)	19,970
Balance	(178,793)	(39,988)	(178,793)	(39,988)
Investment securities: | ASU 2018-02 | Early Adoption
Net of tax
Net income			(15,440)
Cash flow derivative hedges:
Net of tax
Balance	5,302	2,702	3,680	2,184
Other comprehensive income (loss)	131	409	988	927
Balance	$ 5,433	$ 3,111	5,433	$ 3,111
Cash flow derivative hedges: | ASU 2018-02 | Early Adoption
Net of tax
Net income			$ 765