Schedule of Options Granted, Classified into Range of Exercise Price (Detail) - $ / shares		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Outstanding, Number of options and SARs	[2]	729,017	[1]	1,455,908
Outstanding, Weighted average remaining contractual life (years)		5 years 2 months 12 days
Outstanding, Weighted average exercise price		$ 19.77
Exercisable, Number of options and SARs	[2],[3]	513,464
Exercisable, Weighted average remaining contractual life (years)		4 years 8 months 12 days
Exercisable, Weighted average exercise price	[3]	$ 19.07
14.16 - 18.62
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise price, Lower limit		14.16
Range of exercise price, Upper limit		$ 18.62
Outstanding, Number of options and SARs		365,793
Outstanding, Weighted average remaining contractual life (years)		4 years 2 months 12 days
Outstanding, Weighted average exercise price		$ 15.64
Exercisable, Number of options and SARs		290,193
Exercisable, Weighted average remaining contractual life (years)		4 years
Exercisable, Weighted average exercise price		$ 15.80
19.36 - 24.86
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise price, Lower limit		19.36
Range of exercise price, Upper limit		$ 24.86
Outstanding, Number of options and SARs		204,724
Outstanding, Weighted average remaining contractual life (years)		5 years 10 months 25 days
Outstanding, Weighted average exercise price		$ 19.91
Exercisable, Number of options and SARs		142,771
Exercisable, Weighted average remaining contractual life (years)		5 years 9 months 18 days
Exercisable, Weighted average exercise price		$ 19.65
27.17 - 31.51
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise price, Lower limit		27.17
Range of exercise price, Upper limit		$ 31.51
Outstanding, Number of options and SARs		158,500
Outstanding, Weighted average remaining contractual life (years)		6 years 10 months 25 days
Outstanding, Weighted average exercise price		$ 29.13
Exercisable, Number of options and SARs		80,500
Exercisable, Weighted average remaining contractual life (years)		5 years 2 months 12 days
Exercisable, Weighted average exercise price		$ 29.81

[1] Due to the ceiling imposed on the SAR grants, the outstanding amount equals a maximum of 662,075 shares of the Company's common stock issuable upon exercise.

[2] The SAR units are convertible for a maximum number of shares of the Company's common stock equal to 75% of the SAR units subject to the grant.

[3] Due to the ceiling imposed on the SAR grants, the exercisable amount equals a maximum of 465,223 shares of the Company's common stock issuable upon exercise.