Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	$ 2,476,485	$ 2,736,941	$ 2,675,040
Other comprehensive income (loss)	(8,372)	(36,752)	198
Balance	2,532,551	2,476,485	2,736,941
Components of accumulated other comprehensive loss, net of taxes, resulting from other-than-temporarily impaired securities
Non-credit other-than-temporary impairment losses on securities available for sale	0	0
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(88,011)	(51,259)	(51,457)
Other comprehensive income (loss)	(8,372)	(36,752)	198
Balance	(96,383)	(88,011)	(51,259)
Pension and Other Benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(30,237)	(26,883)	(35,869)
Other comprehensive income (loss)	4,291	(3,354)	8,986
Balance	(25,946)	(30,237)	(26,883)
Investment Securities
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(59,958)	(25,106)	(15,533)
Other comprehensive income (loss)	(14,159)	(34,852)	(9,573)
Balance	(74,117)	(59,958)	(25,106)
Cash Flow Derivative Hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	2,184	730	(55)
Other comprehensive income (loss)	1,496	1,454	785
Balance	$ 3,680	$ 2,184	$ 730