Changes and Reclassifications Out of Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 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)			$ (79,274)
Changes in unrealized securities' holding gains/(losses), before Tax	$ 31,489		$ (28,642)		79,868	$ 5,475
Changes in unrealized securities' holding gains/(losses), Tax	11,431		(10,024)		28,994	1,916
Changes in unrealized securities' holding gains/(losses), After-tax	20,058		(18,618)		50,874	3,559
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred into available-for-sale from held-to-maturity, Pre-tax	(221)		(575)		(1,072)	(1,079)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, Tax	(79)		(203)		(14)	(378)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity , Net of Tax	(142)		(372)		(1,058)	(701)
Realized (gains) losses on sale of securities	(2,164)		(567)		(2,459)	(921)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax	(786)		(198)		(892)	(322)
Reclassified amount, net of tax	(1,378)		(369)		(1,567)	(599)
Balance at the beginning and end of the period, Pre-tax	(93,336)	(93,852)	(98,972)	$ (100,520)	(93,336)	(98,972)	$ (95,760)	$ (102,068)
Accumulated other comprehensive income reclassifications, benefit plan, tax	(32,560)	(32,746)	(34,638)	(35,181)	(32,560)	(34,638)	(33,432)	(35,722)
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(60,776)	(61,106)	(64,334)	(65,339)	(60,776)	(64,334)	(62,328)	(66,346)
Net gains/(losses) arising during the period, Pre-Tax	0				0
Net gains/(losses) arising during the period, Tax	0				0
Net gains/(losses) arising during the period, After-tax	0				0
Amortization of actuarial gain, Pre-Tax	1,076		1,138		3,244	2,276
Amortization of actuarial gain, Tax	390		399		1,163	797
Amortization of actuarial losses/(gains), Net of Tax	686		739		2,081	1,479
Amortization of prior service cost reclassified to other noninterest expense, before Tax	(560)		410		(820)	820
Amortization of prior service cost reclassified to other noninterest expense, Tax	(204)		144		(291)	287
Amortization of prior service cost reclassified to other noninterest expense, After-tax	(356)		266		(529)	533
Balance at the end of the Period, Pre-tax	(49,785)		(104,028)		(49,785)	(104,028)
Balance at the end of the period, Tax	(20,312)		(36,407)		(20,312)	(36,407)
Balance at the end of the period, After-tax	(29,473)		(67,621)		(29,473)	(67,621)	(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	14,447	(32,786)	24,728	(8,531)	(32,786)	(8,531)	(8,531)
Balance at the beginning of the period, Tax	1,682	(15,840)	8,656	(2,985)	(15,840)	(2,985)	(2,985)
Balance at the beginning of the period, After-tax	12,765	(16,946)	16,072	(5,546)	(16,946)	(5,546)	(5,546)
Balance at the end of the Period, Pre-tax	43,551	14,447	(5,056)	24,728	43,551	(5,056)	(32,786)	(8,531)
Balance at the end of the period, Tax	12,248	1,682	(1,769)	8,656	12,248	(1,769)	(15,840)	(2,985)
Balance at the end of the period, After-tax	$ 31,303	$ 12,765	$ (3,287)	$ 16,072	$ 31,303	$ (3,287)	$ (16,946)	$ (5,546)