Income Taxes Discrete Items (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Tax Carryforward [Line Items]
Income Tax Reconciliation, Nondeductible Expense, Depletion			$ 111,104,000	$ 53,932,000
Unusual or Infrequent, Tax Effect	8,467,000		4,701,000
Tax Adjustments, Settlements, and Unusual Provisions		6,004,000
State and Local Jurisdiction [Member]
Other Tax Carryforward [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions			1,585,000
Northern Powder River Basin Assets [Member]
Other Tax Carryforward [Line Items]
Unusual or Infrequent, Tax Effect		$ 48,976,000