Debt - Revolving Credit Facility Narrative (Details) $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018	Oct. 30, 2017 USD ($)	Oct. 13, 2017 USD ($)	Dec. 31, 2016 USD ($)
Long-Term Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 9,775	$ 9,775				$ 9,989
Revolving Credit Facility [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity							3,800
Letters of Credit Outstanding, Amount		$ 97	97
Minimum Liquidity Requirement When Covenant Ratio Is At Or Above 1.1 to 1.0			500
Interest Coverage Ratio		1.2 to 1.0
Revolving Credit Facility [Member] | Subsequent Event [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity					$ 3,800
Revolving Credit Facility [Member] | Maximum [Member] | Subsequent Event [Member]
Long-Term Debt Instrument [Line Items]
Additional Indebtedness After Term Loan						$ 1,200
Revolving Credit Facility [Member] | First Lien [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Additional Indebtedness		$ 2,500	2,500
Revolving Credit Facility [Member] | Scenario, Forecast [Member]
Long-Term Debt Instrument [Line Items]
Interest Coverage Ratio	1.25 to 1.0
Revolving Credit Facility [Member] | Scenario, Forecast [Member] | First Lien [Member]
Long-Term Debt Instrument [Line Items]
Leverage Ratio	3.50 to 1.0			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			0.65
Revolving Credit Facility [Member] | Line of Credit [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 645	$ 645				$ 0