Notes Payable - Narrative (Details) $ in Millions	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Fair value of long term debt	$ 940
Excess fair value over carrying value of long-term debt	173
Unamortized discount (premium) and debt Issuance costs, Net	8
Level 2 \| Unsecured Notes
Debt Instrument [Line Items]
Fair value of long term debt	$ 638

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