Commitments and Contingencies (Commitments) (Details 2) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Minimum purchase commitment
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months		$ 25
Unrecorded Unconditional Purchase Obligation, Due within Two Years		17
Unrecorded Unconditional Purchase Obligation, Due within Three Years		13
Unrecorded Unconditional Purchase Obligation, Due within Four Years		11
Unrecorded Unconditional Purchase Obligation, Due within Five Years		21
Thereafter		0
Total	[1]	$ 87
Maximum remaining term under individual purchased power contract (in years)		5 years
Coal, Gas and Transportation Commitments
Commitments and Contingencies
Purchases		$ 1,800	$ 2,600	$ 3,500
Minimum purchase commitment
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months		638
Unrecorded Unconditional Purchase Obligation, Due within Two Years		251
Unrecorded Unconditional Purchase Obligation, Due within Three Years		174
Unrecorded Unconditional Purchase Obligation, Due within Four Years		140
Unrecorded Unconditional Purchase Obligation, Due within Five Years		109
Thereafter		415
Total		$ 1,727

[1] As of December 31, 2016, the maximum remaining term under any individual purchased power contract is five years.