Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	$ 2,476,485	$ 2,736,941
Other comprehensive income (loss)	1,319	32,886
Balance	2,505,994	2,471,734
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(88,011)	(51,259)
Other comprehensive income (loss)	1,319	32,886
Balance	(86,692)	(18,373)
Pension and other benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(30,237)	(26,883)
Balance	(30,237)	(26,883)
Investment securities
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(59,958)	(25,106)
Other comprehensive income (loss)	957	33,388
Balance	(59,001)	8,282
Cash flow derivative hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	2,184	730
Other comprehensive income (loss)	362	(502)
Balance	$ 2,546	$ 228