Net Income Available to Common Stockholders (Detail) - Basic and Diluted Net Income Per Share Available To Common Stockholders (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic:
Net income	$ 9,249	$ 10,833	$ 9,399	$ 9,860	$ 6,994	$ 8,397	$ 10,697	$ 14,695	$ 39,341	$ 40,783	$ 38,957
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic									0	20	502
Net income available to common stockholders – basic	9,249	10,833	9,399	9,860	6,994	8,395	10,691	14,678	39,341	40,763	38,455
Diluted:
Participating Securities, Distributed and Undistributed Earnings (Loss), Diluted									0	0	(19)
Net income available to common stockholders – diluted	$ 9,249	$ 10,833	$ 9,399	$ 9,860	$ 6,994	$ 8,395	$ 10,691	$ 14,678	$ 39,341	$ 40,763	$ 38,474
Basic shares:
Weighted-average shares used in computing basic net income available to comon stockholders									53,444	51,956	49,766
Diluted shares:
Dilutive effect of stock options and restricted stock units using treasury-stock method									1,374	1,696	2,036
Weighted-average shares used in computing diluted net income available to common stockholders									54,818	53,652	51,802
Net income per common share available to common stockholders:
Basic	$ 0.17	$ 0.20	$ 0.18	$ 0.19	$ 0.13	$ 0.16	$ 0.21	$ 0.29	$ 0.74	$ 0.78	$ 0.77
Diluted	$ 0.17	$ 0.20	$ 0.17	$ 0.18	$ 0.13	$ 0.16	$ 0.20	$ 0.28	$ 0.72	$ 0.76	$ 0.74