Long-Term Debt - Schedule of Long Term Debt (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Long-term debt
Total long-term debt, excluding unamortized premiums	$ 10,691	$ 10,705
Unamortized premiums, net	163	185
Total long-term debt	10,854	10,890
Less current maturities	(6)	(8)
Long-term debt, excluding current maturities	$ 10,848	10,882
Senior Notes, 5.75% Due 2022
Long-term debt
Stated interest rate	5.75%
Total long-term debt, excluding unamortized premiums	$ 600	600
Senior Notes with Varied Maturity Date
Long-term debt
Total long-term debt, excluding unamortized premiums	$ 5,315	5,315
Senior Notes with Varied Maturity Date \| Minimum
Long-term debt
Stated interest rate	5.125%
Senior Notes with Varied Maturity Date \| Maximum
Long-term debt
Stated interest rate	6.125%
Tranche B 2024
Long-term debt
Total long-term debt, excluding unamortized premiums	$ 4,611	$ 4,611
Effective percentage	4.432%	3.557%
Tranche B 2024 \| London Interbank Offered Rate (LIBOR)
Long-term debt
Basis spread on variable rate	2.25%
Tranche B 2024 \| London Interbank Offered Rate (LIBOR) \| Minimum
Long-term debt
Basis spread on variable rate	0.00%
Capital Leases
Long-term debt
Total long-term debt, excluding unamortized premiums	$ 165	$ 179

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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