Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 431.9	$ 434.4
Unrealized loss, net	0.4
Unrealized gain, net	0.1
Operating Accounts
Schedule of Investments [Line Items]
Cash, cash equivalents and short-term investments	$ 16.8	$ 19.5