Accumulated Other Comprehensive Loss - Changes, Net of Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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)	$ 19,041	$ 13,146	20,360	46,032
Balance	2,552,602	2,501,008	2,552,602	2,501,008
Accumulated Other Comprehensive Loss
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(86,692)	(18,373)	(88,011)	(51,259)
Other comprehensive income (loss)	19,041	13,146	20,360	46,032
Balance	(67,651)	(5,227)	(67,651)	(5,227)
Pension and other benefits
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	(30,237)	(26,883)	(30,237)	(26,883)
Other comprehensive income (loss)		(46)		(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	(59,001)	8,282	(59,958)	(25,106)
Other comprehensive income (loss)	18,885	13,067	19,842	46,455
Balance	(40,116)	21,349	(40,116)	21,349
Cash flow derivative hedges
Changes in the components of accumulated other comprehensive loss, net of taxes
Balance	2,546	228	2,184	730
Other comprehensive income (loss)	156	125	518	(377)
Balance	$ 2,702	$ 353	$ 2,702	$ 353