Basis of Presentation and Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accounting Policies [Line Items]
Current	$ 128		$ 134
Non-current	28		25
Total	156		159
Interest costs capitalized	7	$ 7
Goodwill	242		242
Debt service
Accounting Policies [Line Items]
Current	12		11
Non-current	7		8
Total	19		19
Construction major maintenance
Accounting Policies [Line Items]
Current	25		28
Non-current	18		16
Total	43		44
Security project insurance
Accounting Policies [Line Items]
Current	88		92
Non-current	0		0
Total	88		92
Other
Accounting Policies [Line Items]
Current	3		3
Non-current	3		1
Total	$ 6		$ 4