Derivatives - Offsetting Assets (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) Counterparty	Dec. 31, 2015 USD ($)
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
Amount of collateral received or pledged related to derivative instruments at period end	$ 0	$ 0
Gross Amounts of Derivative Assets Presented in the Consolidated Balance Sheet	131,000,000	296,000,000
Net Amounts of Derivative Assets	93,000,000	259,000,000
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	131,000,000	296,000,000
Gross Amount of Eligible Offsetting Recognized Derivative Liabilities	38,000,000	37,000,000
Net Amounts of Derivative Assets	$ 93,000,000	$ 259,000,000