Debt Credit Agreement (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 30, 2017
Line of Credit Facility [Line Items]
Loss on extinguishment of debt	$ (73)	$ 0	$ (73)	$ (58)
Amended and Restated Credit Agreement
Line of Credit Facility [Line Items]
Loss on extinguishment of debt			$ (3)
Line of Credit Facility, Additional Borrowing Capacity		1,000
Letters of Credit Issued		$ 7
Amended and Restated Credit Agreement | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Covenant Compliance, Maximum Ratio of Indebtedness to EBITDA		350.00%
Revolving Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Amount drawn		$ 0
Revolving Credit Facility | Revolving Credit Facility | JPMorgan Chase Bank, N.A.
Line of Credit Facility [Line Items]
Revolving Credit Facility, Maximum Borrowing Capacity		$ 2,000			$ 1,500
Revolving Credit Facility | Revolving Credit Facility | JPMorgan Chase Bank, N.A. | LIBOR
Line of Credit Facility [Line Items]
Basis spread of variable rate		1.10%	1.10%
Revolving Credit Facility | Revolving Credit Facility | JPMorgan Chase Bank, N.A. | Administrative Agents Alternate Base Rate
Line of Credit Facility [Line Items]
Basis spread of variable rate		0.10%	0.10%
Tranche A Term Loan, Due 2021 | Loans Payable
Line of Credit Facility [Line Items]
Debt Instrument, Maturity Date		Dec. 31, 2021
Tranche A Term Loan, Due 2021 | JPMorgan Chase Bank, N.A. | Loans Payable
Line of Credit Facility [Line Items]
Borrowings		$ 398
Tranche A Term Loan, Due 2021 | JPMorgan Chase Bank, N.A. | LIBOR | Loans Payable
Line of Credit Facility [Line Items]
Basis spread of variable rate		1.25%	1.25%
Rate effective		2.75%
Tranche A Term Loan, Due 2021 | JPMorgan Chase Bank, N.A. | Administrative Agents Alternate Base Rate | Loans Payable
Line of Credit Facility [Line Items]
Basis spread of variable rate		0.25%	0.25%