Debt Obligations - Trade Receivables Securitization Facility (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
Trade Receivables Securitization Facility, Maximum Borrowing Base	$ 200
Trade Receivables Securitization Facility, Collateral At Period End	$ 505	$ 491
Federal Funds Rate [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.50%