Summary of Convertible Debt (Parenthetical) (Detail)	Jun. 30, 2018
0.75% Senior Subordinated Convertible Notes Due in October 2018
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	0.75%
1.50% Senior Subordinated Convertible Notes Due in October 2020
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%
0.599% Senior Subordinated Convertible Notes Due in August 2024
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	0.599%