Computation of Basic and Diluted Net Income Per Common Share (Parenthetical) (Detail) shares in Millions	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Convertible Notes
Net Income Loss Per Common Share
Initial debt conversion price | $ / shares	$ 31.80
Shares that would be issued upon conversion | shares	14.5
Convertible Notes Hedge Warrant
Net Income Loss Per Common Share
Initial debt conversion price | $ / shares	$ 39.91
Shares that would be issued upon conversion | shares	14.5