Summary of Stock Option Activity (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015
Number of options and SAR units
Number of shares outstanding, Beginning balance	3,316,380
Number of shares, SAR units granted	37,500	[1]
Number of shares, Options/SAR units exercised	(436,806)
Number of shares, Options/SAR units forfeited or expired	(36,594)
Number of shares outstanding, Ending balance	2,880,480	[2]
Number of shares exercisable, Ending balance	1,690,821	[3]
Weighted-average exercise price
Weighted-average exercise price, Beginning balance	$ 16.50
Weighted average exercise price, SAR units granted	$ 19.59	[1]
Weighted average exercise price, Options/SAR units exercised	$ 10.20
Weighted average exercise price, Options/SAR units forfeited or expired	$ 18.62
Weighted average exercise price, Ending balance	$ 17.47	[2]
Weighted average exercise price, exercisable	$ 18.17	[3]
Weighted average remaining contractual term
Weighted average remaining contractual life, Outstanding at end of period	4 years 8 months 12 days	[2]
Weighted average remaining contractual life, Exercisable at end of period	3 years 6 months	[3]
Aggregate intrinsic value
Aggregate intrinsic value, Outstanding	$ 14,188,392
Aggregate intrinsic value, Exercisable	$ 8,100,340

[1] 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.

[2] Due to the ceiling imposed on the SAR grants, the outstanding amount equals a maximum of 2,524,973 shares of the Company's common stock issuable upon exercise.

[3] Due to the ceiling imposed on the SAR grants, the exercisable amount equals a maximum of 1,548,456 shares of the Company's common stock issuable upon exercise.