Fair Value Measurements - Estimated Fair Values of Convertible Notes (Detail) $ in Millions	Dec. 30, 2017 USD ($)
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	$ 285.1
2042 Notes
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	284.8
2043 Notes
Estimated Fair Value Of Financial Instruments [Line Items]
Estimated fair values of debt instruments	$ 0.3