Fair Value Measurements - Estimated Fair Values of Convertible Notes (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	$ 541.7
2012 Notes
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	297.3
2013 Notes
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	$ 244.4