Employee Savings Plan and Employee Stock Purchase Plan - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Contribution Plan Disclosure [Line Items]
401(k) eligibility minimum service period			90 days
401(k) description of plan contributions			contribution for our employees equal to 100% of the first 1% of salary deferrals and 50% of salary deferrals between 2% and 6%, up to a maximum matching contribution of 3.5% of an employee’s salary each plan year.
Employee vested percentage in salary deferrals and roll over contributions			100.00%
Minimum period for vesting 100% contributions			2 years
Minimum period for vesting of discretionary contributions			2 years
Matching contribution amount	$ 1.3	$ 0.9	$ 2.7	$ 2.1
Employee stock purchase plan overlapping offering period			24 months
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Employees Company's common stock shares purchase limit percentage	10.00%		10.00%
Maximum number of shares that may be purchased by a participant			2,000
Share of common stock purchase maximum			2,000,000
Purchase of shares of common stock			33,584	42,937
Compensation expense related to ESPP	$ 0.3	$ 0.2	$ 0.5	$ 0.3
After Two Years Of Employment [Member]
Defined Contribution Plan Disclosure [Line Items]
Matching contributions, vesting percentage			100.00%
One Hundred Percent Match For Percent Of Participants Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer contribution percentage			100.00%
Percentage of salary deferrals			1.00%
50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer contribution percentage			50.00%
Minimum [Member]
Defined Contribution Plan Disclosure [Line Items]
401(k) eligible age of employee			18 years
Minimum [Member] | 50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals			2.00%
Maximum [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals			3.50%
Maximum [Member] | Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Purchase price of common stock expressed as a percentage of its fair market value			85.00%
Maximum [Member] | 50% Matching Contribution [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of salary deferrals			6.00%