Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Equity [Abstract]
Total number of shares authorized	55,000,000		55,000,000
Common stock, shares authorized	50,000,000		50,000,000		50,000,000
Common stock, par value	$ 0.000001		$ 0.000001		$ 0.000001
Preferred stock, shares authorized	5,000,000		5,000,000
Preferred stock, par value	$ 0.000001		$ 0.000001
Common stock, shares issued	23,537,368		23,537,368		23,537,368
Common stock, shares outstanding	23,537,368		23,537,368		23,537,368
Preferred stock, shares issued	0		0		0
Preferred stock, shares outstanding	0		0		0
Maximum number of shares of common stock that may be issued	4,725,551		4,725,551
Shares of common stock reserved for issuance description			Additionally, the number of shares of common stock reserved for issuance under the 2014 Plan will automatically increase on January 1 of each year for a period of up to 10 years, beginning on January 1, 2015 and ending on and including January 1, 2024, by 4% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors.
Shares of common stock reserved for issuance percentage			4.00%
Years of increase in shares of common stock reserved for issuance			10 years
Options granted, Number of Shares	0		0
Options exercised, Number of Shares	0		0
Stock-based compensation expense	$ 0.8	$ 1.5	$ 1.9	$ 2.8
Unrecognized stock-based compensation cost	$ 5.9	$ 15.4	$ 5.9	$ 15.4
Unrecognized stock-based compensation cost, weighted-average period recognized			2 years 5 months 1 day	3 years