Debt Obligations - Trade Receivables Securitization Facility (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Nov. 30, 2014	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Trade receivables securitization facility, maximum borrowing base		$ 200
Trade receivables securitization facility, collateral at period end		$ 540	$ 526
Term of revolving trade receivable securitization facility	2 years
Federal Funds Rate | Secured Debt
Debt Instrument [Line Items]
Debt basis spread on variable rate		0.50%