Investments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Investments Schedule [Abstract]
Realized gain (loss) on marketable securities	$ 1.7	$ (1.8)	$ 1.3	$ (1.9)

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