Summary of Significant Accounting Policies - Other Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies
Other assets including non-controlling investment	$ 1.8	$ 1.8

Original amount of cost-method investments at the acquisition date.

No definition available.

Name:	us-gaap_CostMethodInvestmentsOriginalCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant