Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (117,287)	$ (103,047)
Income tax (expense) or benefit	(636)	(510)
Other comprehensive income (loss), net of tax	1,116	867
Accumulated other comprehensive income (loss), ending balance	(116,171)	(102,180)
Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	38	75
Income tax (expense) or benefit	5	5
Other comprehensive income (loss), net of tax	(8)	(9)
Accumulated other comprehensive income (loss), ending balance	30	66
Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(121,602)	(110,626)
Income tax (expense) or benefit	(1,116)	(988)
Other comprehensive income (loss), net of tax	1,958	1,682
Accumulated other comprehensive income (loss), ending balance	(119,644)	(108,944)
Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	4,277	7,504
Income tax (expense) or benefit	475	473
Other comprehensive income (loss), net of tax	(834)	(806)
Accumulated other comprehensive income (loss), ending balance	3,443	6,698
Actuarial gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,178	2,778
Actuarial gain [Member] | Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,074	2,670
Actuarial gain [Member] | Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	104	108
Negative prior service cost [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(1,413)	(1,387)
Negative prior service cost [Member] | Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(1,413)	(1,387)
Hedging gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(13)	(14)
Hedging gain [Member] | Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	$ (13)	$ (14)