Other Comprehensive Income/Loss (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net unrealized loss on securities available-for-sale before tax amount:
Net unrealized holding gain (loss) on securities arising during the year	$ 14,134	$ (16,202)	$ (2,040)
Reclassification adjustment for gain (loss) on sales of securities included in net income	(141)	(2,057)	(8,477)
Net unrealized gain (loss) on securities available-for-sale	13,993	(18,259)	(10,517)
Net actuarial loss adjustment on pension plans and other postretirement benefits before tax amount:
Net actuarial loss adjustment arising during the year	(60,583)	68,150	14,141
Reclassification adjustment for net actuarial loss included in net income	913	3,610	5,447
Net actuarial loss adjustment on pension plans and other postretirement benefits	(59,670)	71,760	19,588
Prior service cost adjustment on pension plans and other postretirement benefits before tax amount:
Prior service cost adjustment arising during the year			(5,463)
Reclassification adjustment for prior service cost included in net income			152
Prior service cost adjustment on pension plans and other postretirement benefits	190	213	(5,311)
Reclassification adjustment for loss on cash flow hedge included in net income, before tax amount			261
Total other comprehensive income, before tax amount	(45,487)	53,714	4,021
Net unrealized loss on securities available-for-sale, tax benefit (expense):
Net unrealized holding gain (loss) on securities arising during the year	(4,991)	5,717	720
Reclassification adjustment for gain (loss) included in net income	50	725	2,987
Net unrealized gain (loss) on securities available-for-sale	(4,941)	6,442	3,707
Net actuarial loss adjustment on pension plans and other postretirement benefits, tax benefit (expense):
Net actuarial loss adjustment arising during the year	23,116	(23,970)	(4,998)
Reclassification adjustment for net actuarial loss included in net income	(322)	(1,275)	(1,920)
Net actuarial loss adjustment on pension plans and other postretirement benefits	22,794	(25,245)	(6,918)
Prior service cost adjustment on pension plans and other postretirement benefits, tax benefit (expense):
Prior service cost adjustment arising during the year			1,925
Reclassification adjustment for prior service cost included in net income			(54)
Prior service cost adjustment on pension plans and other postretirement benefits	(67)	(71)	1,871
Reclassification adjustment for loss on cash flow hedge included in net income, tax benefit (expense)			(110)
Total other comprehensive income, tax benefit (expense)	17,786	(18,874)	(1,450)
Net unrealized loss on securities available-for-sale net of tax:
Net unrealized holding gain (loss) on securities arising during the year	9,143	(10,485)	(1,320)
Reclassification adjustment for gain (loss) included in net income	(91)	(1,332)	(5,490)
Net unrealized gain (loss) on securities available-for-sale	9,052	(11,817)	(6,810)
Net actuarial loss adjustment on pension plans and other postretirement benefits net of tax:
Net actuarial loss adjustment arising during the year	(37,467)	44,180	9,143
Reclassification adjustment for net actuarial loss included in net income	591	2,335	3,527
Net actuarial loss adjustment on pension plans and other postretirement benefits	(36,876)	46,515	12,670
Prior service cost adjustment on pension plans and other postretirement benefits net of tax:
Prior service cost adjustment arising during the year	0	0	(3,538)
Reclassification adjustment for prior service cost included in net income	123	142	98
Prior service cost adjustment on pension plans and other postretirement benefits	123	142	(3,440)
Reclassification adjustment for loss on cash flow hedge included in net income	0	0	151
Total other comprehensive (loss) income, net of tax	$ (27,701)	$ 34,840	$ 2,571