Summary of Significant Accounting Policies - Income Taxes (Details) $ in Millions	Dec. 31, 2017 USD ($)
ASU 2016-16 \| Proforma Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effect of Adoption	$ (8)

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

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