Debt - Standby Letters of Credit (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Letters of credit, amount outstanding	$ 86	$ 79
JP Morgan Chase
Debt Instrument [Line Items]
Letters of credit, amount outstanding	20	20
Deutsche Bank
Debt Instrument [Line Items]
Letters of credit, amount outstanding	$ 66	$ 59