Fair Value Measurements of Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Long-term Debt	$ 2,289	$ 2,349
Project Debt	165	174
Estimate of Fair Value Measurement [Member]
Assets, Fair Value Disclosure	2	2
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	12
Long-term Debt	2,284	2,371
Project Debt	168	179
Project Debt, Held for Sale	0	510
Reported Value Measurement [Member]
Long-term Debt	2,289	2,349
Project Debt	165	174
Project Debt, Held for Sale	0	510
Fair Value, Inputs, Level 1 [Member] | Mutual And Bond Funds [Member] | Estimate of Fair Value Measurement [Member]
Investments, Fair Value Disclosure	2	2
Energy Hedges [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative Liability	$ 0	$ 5