Merger (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	$ 568
Above Market Contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	829
Above Market Contracts | Energy contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	189
Amortization period of intangible assets and liabilities (in years)	10
Above Market Contracts | NUG contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	124
Amortization period of intangible assets and liabilities (in years)	25
Above Market Contracts | Coal supply contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	516
Amortization period of intangible assets and liabilities (in years)	8
Below Market Contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	261
Below Market Contracts | NUG contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	143
Amortization period of intangible assets and liabilities (in years)	13
Below Market Contracts | Coal supply contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	83
Amortization period of intangible assets and liabilities (in years)	7
Below Market Contracts | Transportation contract
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Valuation of intangible assets and liabilities	$ 35
Amortization period of intangible assets and liabilities (in years)	8