Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (115,055)	$ (101,313)	$ (117,287)	$ (103,047)
Income tax (expense) or benefit	(636)	(510)	(1,908)	(1,530)
Other comprehensive income (loss), net of tax	1,117	867	3,349	2,601
Accumulated other comprehensive income (loss), ending balance	(113,938)	(100,446)	(113,938)	(100,446)
Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	22	57	38	75
Income tax (expense) or benefit	5	5	15	15
Other comprehensive income (loss), net of tax	(7)	(9)	(23)	(27)
Accumulated other comprehensive income (loss), ending balance	15	48	15	48
Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(117,686)	(107,262)	(121,602)	(110,626)
Income tax (expense) or benefit	(1,116)	(988)	(3,348)	(2,964)
Other comprehensive income (loss), net of tax	1,958	1,682	5,874	5,047
Accumulated other comprehensive income (loss), ending balance	(115,728)	(105,579)	(115,728)	(105,579)
Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	2,609	5,892	4,277	7,504
Income tax (expense) or benefit	475	473	1,425	1,419
Other comprehensive income (loss), net of tax	(834)	(806)	(2,502)	(2,419)
Accumulated other comprehensive income (loss), ending balance	1,775	5,085	1,775	5,085
Actuarial gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,178	2,778	9,534	8,335
Actuarial gain [Member] | Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,074	2,670	9,222	8,011
Actuarial gain [Member] | Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	104	108	312	324
Negative prior service cost [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(1,413)	(1,387)	(4,239)	(4,162)
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)	(4,239)	(4,162)
Hedging gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(12)	(14)	(38)	(42)
Hedging gain [Member] | Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	$ (12)	$ (14)	$ (38)	$ (42)