Fair Value Measurements (Carrying Amount and Estimated Fair Values of Financial Instruments) (Details) - USD ($)		1 Months Ended			12 Months Ended
	Apr. 26, 2018	Dec. 31, 2018	Apr. 30, 2018	Jul. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, net		$ 52,000,000			$ 52,000,000	$ 77,000,000
Repayments of long-term debt					2,095,000,000	1,139,000,000	$ 1,175,000,000
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		201,000,000			201,000,000	916,000,000
Derivative instruments, net		52,000,000			52,000,000	77,000,000
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		201,000,000			201,000,000	916,000,000
Derivative instruments, net		52,000,000			52,000,000	$ 77,000,000
Long-term Debt [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt, maturity date						Dec. 31, 2020
Repayments of long-term debt	$ 1,191,000,000		$ 1,191,000,000
Long-term Debt [Member] | Carrying Amount [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans						$ 1,191,000,000
Long-term Debt [Member] | Fair Value [Member] | 2016 Term Loan due December 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans						1,191,000,000
Senior Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Repayments of long-term debt		900,000,000		$ 700,000,000
Senior Notes [Member] | Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		2,342,000,000			2,342,000,000	3,242,000,000
Senior Notes [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes		$ 2,190,000,000			$ 2,190,000,000	3,358,000,000
2018 Credit Facility [Member] | Long-term Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Credit facility, maturity date					Apr. 26, 2023
Natural Gas [Member] | Purchased Call Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid						1,000,000
Natural Gas [Member] | Purchased Call Options [Member] | Not Designated as Hedging Instrument [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid						$ 1,000,000