Derivatives - Offsetting Assets (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Counterparty	Dec. 31, 2014 USD ($)
Offsetting of Financial Assets under Master Netting Agreements with Derivative Counterparties
Number of master netting agreements supported by primary counterparty's parent guarantee | Counterparty	16
Number of credit support agreements by primary counterparty | Counterparty	15
Gross Amounts of Derivative Assets Presented in the Consolidated Balance Sheet	$ 368	$ 256
Net Amounts of Derivative Assets	340	236
Derivatives Subject to Master Netting Agreements
Offsetting of Financial Assets under Master Netting Agreements with Derivative Counterparties
Gross Amounts of Derivative Assets Presented in the Consolidated Balance Sheet	368	256
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	28	20
Net Amounts of Derivative Assets	$ 340	$ 236