Derivatives - Offsetting Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 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	$ 238	$ 197
Net Amount of Derivative Liabilities	212	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	238	197
Gross Amount of Eligible Offsetting Recognized Derivative Assets	26	27
Net Amount of Derivative Liabilities	$ 212	$ 170