Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 148.8	$ 113.0
Restricted cash	8.5
Unrealized loss, net	0.1	0.1
Operating Accounts
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 69.2	$ 51.2