Fair Value Measurements (Details 2-Nonrecurring) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Financial Statement [Line Items]
Fair value of long-lived assets held for use, quarter	$ 0	$ 75
Impairment of long-lived assets held for use	38	262
Assets Held and Used Powder River Basin [Member]
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Financial Statement [Line Items]
Impairment of long-lived assets held for use	15
Assets Held and Used Ozona [Member]
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Financial Statement [Line Items]
Impairment of long-lived assets held for use	$ 21	$ 261