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Investments and Restricted Cash and Investments (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Investments [Abstract]    
Investments $ 1,748 $ 1,393
Equity Method Investments [Abstract]    
Equity method investments 1,103 868
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 653 657
Investments, including equity method and restricted cash and investments, current 137 115
Investments, including equity method and restricted cash and investments, noncurrent 3,367 2,803
Investments, including equity method and restricted cash and investments 3,504 2,918
Quad Cities Station nuclear decommissioning trust funds [Member]    
Restricted Cash and Investments [Abstract]    
Decommissioning Fund Investments, Fair Value 429 424
Solar Star and Topaz Projects [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 95 66
Restricted Cash and Investments, Other [Member]    
Restricted Cash and Investments [Abstract]    
Restricted cash and investments 129 167
Electric Transmission Texas, LLC [Member]    
Equity Method Investments [Abstract]    
Equity method investments $ 585 $ 515
Equity method investment, ownership percentage 50.00% 50.00%
Bridger Coal Company [Member]    
Equity Method Investments [Abstract]    
Equity method investments $ 190 $ 192
Equity method investment, ownership percentage 66.67% 66.67%
Tax Equity Investments    
Equity Method Investments [Abstract]    
Equity method investments $ 168 $ 0
Other equity method investments [Member]    
Equity Method Investments [Abstract]    
Equity method investments 160 161
BYD Company Limited common stock [Member]    
Investments, Including Equity Method And Restricted Cash And Investments [Line Items]    
Available-for-sale Securities, Change in Net Unrealized Holding Gain (Loss) before Taxes 1,000 649
Investments [Abstract]    
Available-for-sale Securities, Equity Securities 1,238 881
Rabbi trusts [Member]    
Investments [Abstract]    
Life Insurance, Corporate or Bank Owned, Amount 380 386
Other investments [Member]    
Investments [Abstract]    
Other Investments $ 130 $ 126