Equity Incentive Plans - Summary of Nonvested Restricted Stock and Restricted Stock Units Activity (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Nonvested, Beginning Balance	1,380	1,268	1,169
Number of Shares, Granted	1,104	931	764
Number of Shares, Vested	(844)	(685)	(614)
Number of Shares, Forfeited or Cancelled	(83)	(134)	(51)
Number of Shares Nonvested Ending Balance	1,557	1,380	1,268
Weighted-Average Grant Date Fair Value, Beginning Balance	$ 6.73	$ 6.39	$ 4.15
Weighted-Average Grant Date Fair Value, Granted	7.31	6.78	8.25
Weighted-Average Grant Date Fair Value, Vested	6.64	6.28	4.91
Weighted-Average Grant Date Fair Value, Forfeited or Cancelled	7.21	6.25	4.52
Weighted-Average Grant Date Fair Value, Ending Balance	$ 7.18	$ 6.73	$ 6.39