Debt - Revolving Credit Facility Narrative (Details) $ in Millions	3 Months Ended			6 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2018	Dec. 31, 2016 USD ($)
Long-Term Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 9,710	$ 9,710		$ 9,989
Revolving Credit Facility [Member]
Long-Term Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity			3,800	3,800		3,800
Letters of Credit Outstanding, Amount			$ 100	100
Minimum Liquidity Requirement When Covenant Ratio Is At Or Above 1.1 to 1.0				500
Interest Coverage Ratio			0.70 to 1.0
Revolving Credit Facility [Member] | Maximum [Member]
Long-Term Debt Instrument [Line Items]
Additional Indebtedness After Term Loan			$ 1,000	1,000
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	1.2 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			$ 575	$ 575		$ 0