Changes and Reclassifications Out of Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Balance at the beginning of the period, After-tax	$ (79,274)
Changes in unrealized securities' holding gains/(losses), before Tax	48,379	$ 34,117
Changes in unrealized securities' holding gains/(losses), Tax	17,562	11,941
Changes in unrealized securities' holding gains/(losses), After-tax	30,817	22,176
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Pre-tax	(1,441)	(504)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, Tax	(147)	(176)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity , Net of Tax	(1,294)	(328)
Realized (gains) losses on sale of securities	295	(354)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax	107	(124)
Reclassified amount, net of tax	188	(230)
Balance at the beginning and end of the period, Pre-tax	(93,852)	(100,520)	$ (95,760)	$ (102,068)
Accumulated other comprehensive income reclassifications, benefit plan, tax	(32,746)	(35,181)	33,432	35,722
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(61,106)	(65,339)	(62,328)	(66,346)
Net gains/(losses) arising during the period, Pre-Tax	0
Net gains/(losses) arising during the period, Tax	0
Net gains/(losses) arising during the period, After-tax	0
Amortization of actuarial gain, Pre-Tax	2,168	1,138
Amortization of actuarial gain, Tax	773	398
Amortization of actuarial losses/(gains), Net of Tax	1,395	740
Amortization of prior service cost reclassified to other noninterest expense, before Tax	(260)	410
Amortization of prior service cost reclassified to other noninterest expense, Tax	(87)	143
Amortization of prior service cost reclassified to other noninterest expense, After-tax	(173)	267
Balance at the end of the Period, Pre-tax	(79,405)	(75,792)
Balance at the end of the period, Tax	(31,064)	(26,525)
Balance at the end of the period, After-tax	(48,341)	(49,267)	(79,274)
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	(32,786)	(8,531)	(8,531)
Balance at the beginning of the period, Tax	(15,840)	(2,985)	(2,985)
Balance at the beginning of the period, After-tax	(16,946)	(5,546)	(5,546)
Balance at the end of the Period, Pre-tax	14,447	24,728	(32,786)	(8,531)
Balance at the end of the period, Tax	1,682	8,656	(15,840)	(2,985)
Balance at the end of the period, After-tax	$ 12,765	$ 16,072	$ (16,946)	$ (5,546)