Debt - Revolving Credit Facility Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Oct. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Long-Term Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 9,981		$ 9,989
Revolving Credit Facility [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity			$ 3,800	3,800
Letters of Credit Outstanding, Amount		$ 116
Interest Coverage Ratio		1.25 to 1.0
Revolving Credit Facility [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Additional Indebtedness After Term Loan		$ 1,300
Revolving Credit Facility [Member] | First Lien [Member]
Long-Term Debt Instrument [Line Items]
Leverage Ratio		3.50 to 1.0
Revolving Credit Facility [Member] | First Lien [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital		0.65
Revolving Credit Facility [Member] | Scenario, Forecast [Member] | First Lien [Member]
Long-Term Debt Instrument [Line Items]
Leverage Ratio	3.00 to 1.0
Revolving Credit Facility [Member] | Scenario, Forecast [Member] | First Lien [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital	0.65
Revolving Credit Facility [Member] | Line of Credit [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 781		$ 0