Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at Beginning of Period	$ 425		$ 404
Accretion expense	35		32
Changes in estimate of cash flow or settlement date	25	[1]	(16)
Obligations assumed in RJS Power acquisition	18		0
Obligations incurred	2		13
Obligations settled	(4)		(8)
Balance at End of Period	501		$ 425
Changes in estimate of cash flow or settlement date, new CCR rule	$ 41

[1] Includes increases in 2015 of $41 million as a result of a new CCR rule. Further changes to the AROs may be required as estimates are refined and analysis of the rule continues.