Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Pension Plans [Member]	Dec. 31, 2011 Pension Plans [Member]	Dec. 31, 2010 Pension Plans [Member]	Dec. 31, 2012 Post-Retirement Plans [Member]	Dec. 31, 2011 Post-Retirement Plans [Member]	Dec. 31, 2010 Post-Retirement Plans [Member]	Dec. 31, 2012 Derivative [Member]	Dec. 31, 2011 Derivative [Member]	Dec. 31, 2010 Derivative [Member]	Dec. 31, 2009 Derivative [Member]
Schedule of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive income (loss), beginning balance	$ (103,047)	$ (46,234)	$ (46,389)	$ (110,626)	$ (54,325)	$ (43,103)	$ 7,504	$ 10,761	$ (1,224)	$ 38	$ 75	$ (2,670)	$ (2,062)
Other comprehensive income (loss) before reclassifications:
Actuarial gain (loss) for retiree liabilities	(27,350)	(91,523)	2,974	(27,518)	(91,715)	(19,685)	168	192	22,659
Unrealized gain (loss) for derivative instruments	0	631	(848)
Amounts reclassified from accumulated other comprehensive income:
Actuarial gain (loss)	11,114	2,911	2,389	10,681	2,700	2,068	433	211	321
Negative prior service cost	(5,552)	(5,552)	(4,168)				(5,552)	(5,552)	(4,168)
Hedging gain	(57)	(223)	(106)
Unrealized loss on derivative instruments		3,932
Income Tax (Expense) or Benefit				5,861	32,714	6,395	1,724	1,892	(6,827)
Income Tax (Expense) or Benefit	20	(1,595)	346
Income Tax (Expense) or Benefit	7,605	33,011	(86)
Other comprehensive income (loss), net of tax				(10,976)	(56,301)	(11,222)	(3,227)	(3,257)	11,985
Other comprehensive income (loss), net of tax	(37)	2,745	(608)
Other comprehensive income (loss), net of tax	(14,240)	(56,813)	155
Accumulated other comprehensive income (loss), ending balance	$ (117,287)	$ (103,047)	$ (46,234)	$ (121,602)	$ (110,626)	$ (54,325)	$ 4,277	$ 7,504	$ 10,761	$ 38	$ 75	$ (2,670)	$ (2,062)