Derivative Instruments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Total gain (loss) to be reclassified during next 12 months	$ (58)
Margin cash collateral received from counterparties that was not offset against derivative assets	20	$ 60
Margin cash collateral provided to counterparties that was not offset against derivative assets or liabilities	$ 238	$ 122