Derivatives - Offsetting Assets (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) Counterparty	Dec. 31, 2018 USD ($)
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	$ 171	$ 146
Net Amount of Derivative Assets	132	108
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	171	146
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	39	38
Net Amount of Derivative Assets	$ 132	$ 108