Statement of Comprehensive Income Statement - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Income before income tax expense	$ 326,503	$ 339,894	$ 263,191
Income tax expense	97,019	101,943	79,507
Net income	229,484	237,951	183,684
Changes in unrealized securities' holding gains/(losses), Pre-Tax		38,864
Changes in unrealized securities' holding gains/(losses), Tax		13,602
Changes in unrealized securities' holding gains/(losses), After-tax		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		(2,157)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Tax		(753)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, After-tax		(1,404)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Pre-Tax		(166)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax		(58)
Reclassified amount, net of tax		108
Net change in unrealized gains/(losses) on securities available for sale, Pre-Tax	(24,255)	36,541	(130,331)
Net change in unrealized gains/(losses) on securities available for sale, Tax	(12,855)	12,791	(45,617)
Net change in unrealized gains/(losses) on securities available for sale, After-tax	(11,400)	23,750	(84,714)
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)
Net change from defined benefit pension plans, Pre-Tax	6,308	(44,255)	52,375
Net change from defined benefit pension plans, Tax	2,290	(15,489)	18,332
Net change from defined benefit pension plans, After-tax	4,018	(28,766)	34,043
Total other comprehensive gains/(losses), Pre-Tax	(17,947)	(7,714)	(77,956)
Total other comprehensive gains/(losses), Tax	(10,565)	(2,698)	(27,285)
Total other comprehensive gains/(losses), After-tax	(7,382)	(5,016)	(50,671)
Comprehensive income, Pre-Tax	308,556	332,180	185,235
Comprehensive income, Tax	86,454	99,245	52,222
Comprehensive income, After-tax	$ 222,102	$ 232,935	$ 133,013