Fair Value Measurements (Schedule of Carrying Values and Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 180				$ 180		$ 631
Restricted cash	69				69		74
Less current portion of long-term debt	139				139		450
Long-term debt	$ 2,054				$ 2,054		1,839
Dividends declared per share	$ 0.55	$ 0.55	$ 0	$ 0.32	$ 1.10	$ 0.32
Senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	$ 978				$ 978		1,839
Convertible Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	1,076				1,076		0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Less current portion of long-term debt	141				141		451
Estimate of Fair Value Measurement [Member] | Senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	1,125				1,125		1,995
Estimate of Fair Value Measurement [Member] | Convertible Debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt	1,573				1,573		0
Cash
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	180				180		631
Cash and cash equivalents, Fair Value	$ 180				$ 180		$ 631