Fair Value Measurements (Carrying Amount and Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments, net	$ 401	$ 155
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	5	5
Derivative instruments, net	401	155
Carrying Amount | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	2,148	2,228
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	5	5
Derivative instruments, net	401	155
Fair Value | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	1,460	2,085
2018 revolving credit facility due April 2024 | Carrying Amount | Long-term debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2018 revolving credit facility due April 2024	149	34
2018 revolving credit facility due April 2024 | Fair Value | Long-term debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
2018 revolving credit facility due April 2024	149	34
Fixed price swaps | Natural Gas
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Premium paid	$ 9	$ 9