Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 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)	$ 13,146	$ (9,897)	46,032	6,090
Balance	2,501,008	2,715,530	2,501,008	2,715,530
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(18,373)	(35,470)	(51,259)	(51,457)
Other comprehensive income (loss)			46,032	6,090
Balance	(5,227)	(45,367)	(5,227)	(45,367)
Pension and other benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(26,883)	(35,869)	(26,883)	(35,869)
Other comprehensive income (loss)	(46)		(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	8,282	1,067	(25,106)	(15,533)
Other comprehensive income (loss)	13,067	(10,396)	46,455	6,204
Balance	21,349	(9,329)	21,349	(9,329)
Cash flow derivative hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	228	(668)	730	(55)
Other comprehensive income (loss)	125	499	(377)	(114)
Balance	$ 353	$ (169)	$ 353	$ (169)