Changes and Reclassifications Out of Other Comprehensive Income (Changes in AOCI by Components of Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Balance at the beginning of the Period, Pre-tax		$ (110,599)
Balance at the beginning of the period, Tax		(38,707)
Balance at the beginning of the period, After-tax		(71,892)
Changes in unrealized securities' holding gains/(losses), Pre-Tax	$ (130,947)		$ 38,864
Changes in unrealized securities' holding gains/(losses), Tax	(45,833)		13,602
Changes in unrealized securities' holding gains/(losses), After-tax	(85,114)		25,262
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Pre-tax	(2,187)		(2,157)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Tax	(765)		(753)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, After-tax	(1,422)		(1,404)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Pre-Tax	2,803		(166)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax	981		(58)
Reclassified amount, net of tax	(1,822)		108
Balance at the beginning and end of the period, Pre-tax	(102,068)		(102,068)	$ (57,813)
Accumulated other comprehensive income reclassifications, benefit plan, tax			(35,722)	20,233
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(66,346)		(66,346)	(37,580)
Net gains/(losses) arising during the period, Pre-Tax			(49,552)	47,939
Net gains/(losses) arising during the period, Tax			(17,344)	16,779
Net gains/(losses) arising during the period, After-tax			(32,208)	31,160
Amortization of actuarial gain, Pre-Tax			(3,166)	(4,437)
Amortization of actuarial gain, Tax			(1,108)	(1,553)
Amortization of actuarial gain, After-tax			(2,058)	(2,884)
Amortization of prior service cost reclassified to other noninterest expense, Pre-Tax			(2,131)	1
Amortization of prior service cost reclassified to other noninterest expense, Tax			(747)	0
Amortization of prior service cost reclassified to other noninterest expense, After-tax			1,384	(1)
Balance at the end of the Period, Pre-tax	(110,599)	(128,546)	(110,599)
Balance at the end of the period, Tax	(38,707)	(49,272)	(38,707)
Balance at the end of the period, After-tax	(71,892)	(79,274)	(71,892)
Unrealized and realized securities gains and losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Balance at the beginning of the Period, Pre-tax	(8,531)	(8,531)	(45,072)
Balance at the beginning of the period, Tax	(2,985)	(2,985)	(15,776)
Balance at the beginning of the period, After-tax	(5,546)	(5,546)	(29,296)
Changes in unrealized securities' holding gains/(losses), Pre-Tax		(19,842)
Changes in unrealized securities' holding gains/(losses), Tax		(11,253)
Changes in unrealized securities' holding gains/(losses), After-tax		(8,589)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Pre-tax		(3,456)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Tax		(1,254)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, After-tax		(2,202)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Pre-Tax		(957)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax		(348)
Reclassified amount, net of tax		609
Balance at the end of the Period, Pre-tax	(8,531)	(32,786)	(8,531)	(45,072)
Balance at the end of the period, Tax	(2,985)	(15,840)	(2,985)	(15,776)
Balance at the end of the period, After-tax	(5,546)	(16,946)	(5,546)	$ (29,296)
Pension plans and other postretirement benefits
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Balance at the beginning and end of the period, Pre-tax	(102,068)	(95,760)	(102,068)
Accumulated other comprehensive income reclassifications, benefit plan, tax		(33,432)	35,722
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	$ (66,346)	(62,328)	$ (66,346)
Net gains/(losses) arising during the period, Pre-Tax		116
Net gains/(losses) arising during the period, Tax		42
Net gains/(losses) arising during the period, After-tax		74
Amortization of actuarial gain, Pre-Tax		(4,552)
Amortization of actuarial gain, Tax		(1,652)
Amortization of actuarial gain, After-tax		(2,900)
Amortization of prior service cost reclassified to other noninterest expense, Pre-Tax		(1,640)
Amortization of prior service cost reclassified to other noninterest expense, Tax		(596)
Amortization of prior service cost reclassified to other noninterest expense, After-tax		$ 1,044