Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance			$ 2,736,941	$ 2,675,040
Other comprehensive income (loss)	$ (5,088)	$ 10,541	40,944	16,631
Balance	2,523,963	2,744,340	2,523,963	2,744,340
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(5,227)	(45,367)	(51,259)	(51,457)
Other comprehensive income (loss)			40,944	16,631
Balance	(10,315)	(34,826)	(10,315)	(34,826)
Pension and other benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(26,929)	(35,869)	(26,883)	(35,869)
Other comprehensive income (loss)			(46)
Balance	(26,929)	(35,869)	(26,929)	(35,869)
Investment securities
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	21,349	(9,329)	(25,106)	(15,533)
Other comprehensive income (loss)	(6,023)	11,036	40,432	17,240
Balance	15,326	1,707	15,326	1,707
Cash flow derivative hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	353	(169)	730	(55)
Other comprehensive income (loss)	935	(495)	558	(609)
Balance	$ 1,288	$ (664)	$ 1,288	$ (664)