Summary of Option Activity (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares
Number of shares, Options outstanding, beginning of year	1,907	2,139	2,034
Number of shares, Granted	335	549	411
Number of shares, Exercised	(359)	(633)	(219)
Number of shares, Expired or Forfeited	(14)	(148)	(87)
Number of shares, Options outstanding, end of year	1,869	1,907	2,139
Number of shares, Options exercisable, end of year	872	776	961
Weighted average exercise price
Weighted average exercise price, Options outstanding, beginning of year	$ 11.54	$ 10.57	$ 9.67
Weighted average exercise price, Granted	21.60	12.20	13.49
Weighted average exercise price, Exercised	10.89	8.66	7.62
Weighted average exercise price, Expired or Forfeited	12.78	12.32	10.72
Weighted average exercise price, Options outstanding, end of year	13.46	11.54	10.57
Weighted average exercise price, Options exercisable, end of year	$ 11.11	$ 10.65	$ 9.07