Merger (Details 1) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	$ 568
Above Market Contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	829
Above Market Contracts | Energy contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	189
Weighted Average Amortization Period (in years)	10 years
Above Market Contracts | NUG contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	124
Weighted Average Amortization Period (in years)	25 years
Above Market Contracts | Coal supply contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	516
Weighted Average Amortization Period (in years)	8 years
Below Market Contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	261
Below Market Contracts | NUG contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	143
Weighted Average Amortization Period (in years)	13 years
Below Market Contracts | Coal supply contracts
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	83
Weighted Average Amortization Period (in years)	7 years
Below Market Contracts | Transportation contract
The preliminary valuation of the additional intangible assets and liabilities recorded as result of the merger
Preliminary Valuation	$ 35
Weighted Average Amortization Period (in years)	8 years