Supplementary Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Prepaid Expense, Current	$ 28	$ 37
Derivative Instruments and Hedges, Assets	3	25
Spare Parts, Assets	21	17
Renewable Energy Credits, Assets	3	15
Other Assets, Current	17	23
Prepaid Expense and Other Assets, Current	72	117
Accrued Operating Expenses Payroll And Related Expenses Current	164	114
Deferred Revenue	16	13
Accrued Liabilities To Client Communities Current	19	22
Interest Payable, Current	30	24
Dividends Payable, Current	35	34
Other Liabilities, Current	25	27
Accrued expenses and other current liabilities	$ 289	$ 234