Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (116,171)	$ (102,180)	$ (117,287)	$ (103,047)
Income tax (expense) or benefit	(636)	(510)	(1,272)	(1,020)
Other comprehensive income (loss), net of tax	1,116	867	2,232	1,734
Accumulated other comprehensive income (loss), ending balance	(115,055)	(101,313)	(115,055)	(101,313)
Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	30	66	38	75
Income tax (expense) or benefit	5	5	10	10
Other comprehensive income (loss), net of tax	(8)	(9)	(16)	(18)
Accumulated other comprehensive income (loss), ending balance	22	57	22	57
Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	(119,644)	(108,944)	(121,602)	(110,626)
Income tax (expense) or benefit	(1,116)	(988)	(2,232)	(1,976)
Other comprehensive income (loss), net of tax	1,958	1,682	3,916	3,364
Accumulated other comprehensive income (loss), ending balance	(117,686)	(107,262)	(117,686)	(107,262)
Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), beginning balance	3,443	6,698	4,277	7,504
Income tax (expense) or benefit	475	473	950	946
Other comprehensive income (loss), net of tax	(834)	(806)	(1,668)	(1,612)
Accumulated other comprehensive income (loss), ending balance	2,609	5,892	2,609	5,892
Actuarial gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,178	2,778	6,356	5,556
Actuarial gain [Member] | Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	3,074	2,670	6,148	5,340
Actuarial gain [Member] | Post-Retirement Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	104	108	208	216
Negative prior service cost [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(1,413)	(1,387)	(2,826)	(2,774)
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)	(2,826)	(2,774)
Hedging gain [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	(13)	(14)	(26)	(28)
Hedging gain [Member] | Derivative [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from accumulated other comprehensive income	$ (13)	$ (14)	$ (26)	$ (28)