Derivatives - Offsetting Assets (Details) $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) Counterparty	Dec. 31, 2016 USD ($)
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	$ 90	$ 259
Net Amounts of Derivative Assets	55	220
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	90	259
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	35	39
Net Amounts of Derivative Assets	$ 55	$ 220