Summary of Significant Accounting Policies - Short-Term Investments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Short-term Investments
Unrealized gains (losses) on available-for-sale securities	$ 0.0	$ 0.0

Amount before tax of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

No definition available.

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