Investments - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments Schedule [Abstract]
Realized gain (loss) on marketable securities	$ 0.6	$ 0.3	$ 1.9	$ (1.6)

Amount of realized gain (loss) on investment in debt and equity securities.

No definition available.

Name:	us-gaap_DebtAndEquitySecuritiesRealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration