Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 95.3	$ 200.7
Unrealized loss, net	0.5	0.3
Operating Accounts
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 47.0	$ 97.8