Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Measurement Inputs, Disclosure [Line Items]
Total cash and cash equivalents	$ 106,092	$ 105,703	$ 88,509	$ 89,955
Long-term restricted cash	1,181	1,181
Fair Value, Inputs, Level 1
Fair Value, Measurement Inputs, Disclosure [Line Items]
Cash	49,452	49,495
Money market funds	56,640	56,208
Total cash and cash equivalents	106,092	105,703
Short-term investments	51	63
Long-term restricted cash	1,181	1,181
Total cash and cash equivalents and investments	$ 107,324	$ 106,947
Performance based restricted stock
Fair Value, Measurement Inputs, Disclosure [Line Items]
Granted (shares)	0
Chief Executive Officer | Performance based restricted stock
Fair Value, Measurement Inputs, Disclosure [Line Items]
Granted (shares)		31,250	31,250