Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 200.7	$ 113.0
Unrealized losses (net of unrealized gains), net of tax, related to temporary decrease in value of available-for-sale securities	0.3
Operating Accounts
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 97.8	$ 51.2