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

Debt - Debt Fair Value (Details) - Fair value measurements using significant other observable inputs (Level 2) - Unsecured Notes - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
2023 Notes
Debt Instrument [Line Items]
Fair value of notes	$ 818	$ 795
New 2021 Notes
Debt Instrument [Line Items]
Fair value of notes	635
2026 Notes
Debt Instrument [Line Items]
Fair value of notes	$ 808
Original 2021 Notes
Debt Instrument [Line Items]
Fair value of notes		$ 1,571