Changes and Reclassifications Out of Accumulated Other Comprehensive Income (Schedule of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended		12 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost reclassified to other noninterest expense, before Tax	$ 410		$ 516		$ 1,230	$ 1,614
Amortization of prior service cost reclassified to other noninterest expense, Tax	144		181		431	565
Amortization of prior service cost reclassified to other noninterest expense, Net of Tax	266		335		799	1,049
Accumulated Other Comprehensive Income (Loss), before Tax	(78,096)		(76,280)		(78,096)	(76,280)
accumulated other comprehensive income, tax	(27,330)		(26,697)		(27,330)	(26,697)
Accumulated other comprehensive loss	(50,766)		(49,583)		(50,766)	(49,583)	$ (71,892)
Changes in unrealized securities' holding gains/(losses), before Tax	25,691		(16,207)		31,166	24,293
Changes in unrealized securities' holding gains/(losses), Tax	8,991		(5,672)		10,908	8,503
Changes in unrealized securities' holding gains/(losses), Net of Tax	16,700		(10,535)		20,258	15,790
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, before Tax	(1,266)		(563)		(2,345)	(1,551)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity, Tax	(442)		(197)		(821)	(543)
Changes in unrealized securities' holding gains/(losses) that result from securities being transferred from available-for-sale to held-to-maturity , Net of Tax	(824)		(366)		(1,524)	(1,008)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, before Tax	(41)		(14)		(962)	(150)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Tax	(14)		(5)		(336)	(53)
Net losses/(gains) realized on sale of securities reclassified to noninterest income, Net of Tax	(27)		(9)		(626)	(97)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax	(97,424)		(53,800)		(97,424)	(53,800)	(102,068)	$ (57,812)	$ (98,972)	$ (55,084)
Accumulated other comprehensive income reclassifications, benefit plan, tax	(34,096)	$ (34,639)	(18,829)	$ (19,279)	(34,096)	(18,829)	(35,722)	(20,233)
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(63,328)		(34,971)		(63,328)	(34,971)	(66,346)	(37,579)	(64,333)	(35,805)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0		0		0	0
Amortization of actuarial losses/(gains), before Tax	1,138		768		3,414	2,398
Amortization of actuarial losses/(gains), Tax	399		269		1,195	839
Amortization of actuarial losses/(gains), Net of Tax	739		499		2,219	1,559
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	0		0		0	0
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Tax	0		0		0	0
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), before Tax	19,328		(22,480)		19,328	(22,480)	(8,531)	(45,072)	(5,056)	(5,696)
accumulated other comprehensive income, tax	6,766		(7,868)		6,766	(7,868)	(2,985)	(15,775)	(1,769)	(1,994)
Accumulated other comprehensive loss	$ 12,562		$ (14,612)		$ 12,562	$ (14,612)	$ (5,546)	$ (29,297)	$ (3,287)	$ (3,702)