Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pre-tax Amount
Balance at beginning of period	$ (30,369)	$ (58,623)	$ (84,722)	$ (85,048)
Other comprehensive income	21,725	(16,356)	76,078	10,069
Balance at end of period	(8,644)	(74,979)	(8,644)	(74,979)
Income Tax Benefit (Expense)
Balance at beginning of period	11,996	23,153	33,463	33,591
Other comprehensive income	(8,579)	6,459	(30,046)	(3,979)
Balance at end of period	3,417	29,612	3,417	29,612
Net of tax
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
Net of tax
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
Pre-tax Amount
Unrealized net gains (losses) arising during the period	(78)		(78)
Other comprehensive income	(78)		(78)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	32		32
Other comprehensive income	32		32
Net of tax
Balance	(26,883)	(35,869)	(26,883)	(35,869)
Unrealized net gains (losses) arising during the period	(46)		(46)
Other comprehensive income (loss)	(46)		(46)
Balance	(26,929)	(35,869)	(26,929)	(35,869)
Investment securities
Pre-tax Amount
Unrealized net gains (losses) arising during the period	21,605	(11,322)	102,515	21,119
Reclassification of net gains to net income	(3)	(5,859)	(25,731)	(10,862)
Other comprehensive income	21,602	(17,181)	76,784	10,257
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(8,535)	4,471	(40,493)	(8,343)
Reclassification of net gains to net income		2,314	10,164	4,290
Other comprehensive income	(8,535)	6,785	(30,329)	(4,053)
Net of tax
Balance	8,282	1,067	(25,106)	(15,533)
Unrealized net gains (losses) arising during the period	13,070	(6,851)	62,022	12,776
Reclassification of net gains to net income	(3)	(3,545)	(15,567)	(6,572)
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
Pre-tax Amount
Unrealized net gains (losses) arising during the period	201	1,282	(628)	269
Reclassification of net gains to net income		(457)		(457)
Other comprehensive income	201	825	(628)	(188)
Income Tax Benefit (Expense)
Unrealized net gains (losses) arising during the period	(76)	(507)	251	(107)
Reclassification of net gains to net income		181		181
Other comprehensive income	(76)	(326)	251	74
Net of tax
Balance	228	(668)	730	(55)
Unrealized net gains (losses) arising during the period	125	775	(377)	162
Reclassification of net gains to net income		(276)		(276)
Other comprehensive income (loss)	125	499	(377)	(114)
Balance	$ 353	$ (169)	$ 353	$ (169)