Calculation of Basic and Diluted Net Income Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic net income per share:
Net income applicable to common stockholders	$ 28,649	$ 16,239	$ 39,122	$ 23,535
Undistributed earnings allocated to participating securities	0	(88)	0	(189)
Net income applicable to common stockholders excluding amounts applicable to participating securities	$ 28,649	$ 16,151	$ 39,122	$ 23,346
Weighted average common shares outstanding	67,591	68,938	68,158	68,805
Basic net income per share	$ 0.42	$ 0.23	$ 0.57	$ 0.34
Diluted net income per share:
Net income applicable to common stockholders	$ 28,649	$ 16,239	$ 39,122	$ 23,535
Undistributed earnings allocated to participating securities	0	(88)	0	(189)
Net income applicable to common stockholders excluding amounts applicable to participating securities	$ 28,649	$ 16,151	$ 39,122	$ 23,346
Weighted average common shares outstanding	67,591	68,938	68,158	68,805
Dilutive shares from assumed conversion of convertible senior notes	2,047	1,877	1,888	1,734
Dilutive stock options and restricted stock	1,056	742	1,076	785
Diluted weighted average common shares outstanding	70,694	71,557	71,122	71,324
Diluted net income per share	$ 0.41	$ 0.23	$ 0.55	$ 0.33