Derivatives - Offsetting Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Offsetting of Financial Liabilities under Master Netting Agreements with Derivative Counterparties
Gross Amounts of Derivative Liabilities Presented in the Consolidated Balance Sheet	$ 117	$ 197
Net Amount of Derivative Liabilities	58	170
Derivatives Subject to Master Netting Agreements
Offsetting of Financial Liabilities under Master Netting Agreements with Derivative Counterparties
Gross Amounts of Derivative Liabilities Presented in the Consolidated Balance Sheet	115	197
Gross Amount of Eligible Offsetting Recognized Derivative Assets	59	27
Net Amount of Derivative Liabilities	56	$ 170
Derivatives Not Subject to Master Netting Agreements
Offsetting of Financial Liabilities under Master Netting Agreements with Derivative Counterparties
Gross Amounts of Derivative Liabilities Presented in the Consolidated Balance Sheet	2
Net Amount of Derivative Liabilities	$ 2