Derivatives - Offsetting Assets (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) Counterparty	Dec. 31, 2017 USD ($)
Offsetting of Financial Assets under Master Netting Agreements with Derivative Counterparties
Number of master netting agreements supported by primary counterparty's parent guarantee | Counterparty	17
Number of credit support agreements by primary counterparty | Counterparty	16
Gross Amounts of Derivative Assets Presented in the Consolidated Balance Sheet	$ 138	$ 78
Net Amounts of Derivative Assets	93	51
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	138	78
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	45	27
Net Amounts of Derivative Assets	$ 93	$ 51