Earnings Per Share - Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic net income (loss) per share:
Net income (loss) available to common shareholders	$ (26,345)	$ 3,454	$ 2,759	$ 4,787	$ 19,949	$ 17,662	$ 5,235	$ 2,103	$ (15,345)	$ 44,949	$ 32,199
Weighted average common shares outstanding									46,281	46,900	47,217
Basic net income (loss) per share	$ (0.58)	$ 0.07	$ 0.06	$ 0.10	$ 0.43	$ 0.38	$ 0.11	$ 0.04	$ (0.33)	$ 0.96	$ 0.68
Dilutive net income (loss) per share:
Net income (loss) available to common shareholders	$ (26,345)	$ 3,454	$ 2,759	$ 4,787	$ 19,949	$ 17,662	$ 5,235	$ 2,103	$ (15,345)	$ 44,949	$ 32,199
Weighted average common shares outstanding									46,281	46,900	47,217
Dilutive stock options, restricted stock, and ESPP purchase rights										897	933
Weighted average common shares outstanding for purposes of computing diluted net income (loss) per share									46,281	47,797	48,150
Dilutive net income (loss) per share	$ (0.58)	$ 0.07	$ 0.06	$ 0.10	$ 0.42	$ 0.37	$ 0.11	$ 0.04	$ (0.33)	$ 0.94	$ 0.67