Nature of Operations and Summary of Significant Accounting Policies - Reconciliation of Shares Used in Computation of Basic and Diluted Net Income Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basic (in shares)	79,070	79,648	80,449
Assumed dilution under stock plans (in shares)	300	333	442
Diluted (in shares)	79,370	79,981	80,891