Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance			$ 2,476,485	$ 2,736,941
Other comprehensive income (loss)	$ 537	$ (5,088)	20,897	40,944
Balance	2,581,858	2,523,963	2,581,858	2,523,963
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(67,651)	(5,227)	(88,011)	(51,259)
Other comprehensive income (loss)	537	(5,088)	20,897	40,944
Balance	(67,114)	(10,315)	(67,114)	(10,315)
Pension and other benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(30,237)	(26,929)	(30,237)	(26,883)
Other comprehensive income (loss)				(46)
Balance	(30,237)	(26,929)	(30,237)	(26,929)
Investment securities
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(40,116)	21,349	(59,958)	(25,106)
Other comprehensive income (loss)	128	(6,023)	19,970	40,432
Balance	(39,988)	15,326	(39,988)	15,326
Cash flow derivative hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	2,702	353	2,184	730
Other comprehensive income (loss)	409	935	927	558
Balance	$ 3,111	$ 1,288	$ 3,111	$ 1,288