Debt and Interest Costs - FV, Maturities, etc. (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long term debt value
Letters of credit, outstanding amount	$ 28	$ 33
Ownership interest in guarantor subsidiaries (as a percent)	100.00%
Leverage ratio, maximum	4.25
Level 2
Long term debt value
Fair value of the long-term debt	$ 7,400	$ 7,000

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

No definition available.

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant