Stock-Based Compensation (Schedule of Stock-Based Compensation Costs) (Details) - Stock Options [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, Compensation Cost [Line Items]
Stock-based compensation cost - general and administrative expense	$ 3	$ 6	$ 5
Stock-based compensation cost - general and administrative expense capitalized	$ 1	1	$ 3
Workforce Reduction [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation cost - general and administrative expense		1
Executive Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation cost - general and administrative expense		$ 1