Stock-Based Compensation (Schedule of Stock-Based Compensation Costs - Equity-Classified Performance Units) (Details) - Equity-Classified Performance Units [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost - general and administrative expense	$ 5	$ 9	$ 6
Stock-based compensation cost - general and administrative expense capitalized	$ 2	1	$ 4
Workforce Reduction [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost - general and administrative expense		1
Executive Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation cost - general and administrative expense		$ 1