Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain associated with Maine fossil	$ 0	$ 0	$ 21
Maine fossil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain associated with Maine fossil		21
After-tax gain (loss) on assets previously held for sale		$ 12