Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - USD ($) $ in Millions		1 Months Ended
	Apr. 27, 2017	Apr. 30, 2017
Collaboration and License Agreement [Member] | GE Healthcare Limited [Member]
Subsequent Event [Line Items]
Upfront cash payment received		$ 5.0
Regulatory and sales milestones payments expected		$ 60.0
2015 Equity Incentive Plan [Member]
Subsequent Event [Line Items]
Share based compensation, additional shares authorized	1,200,000
Share based compensation, shares authorized	5,755,277
2017 Employee Stock Purchase Plan [Member]
Subsequent Event [Line Items]
Share based compensation, shares authorized	250,000
Percentage of discount on closing price	15.00%
Employee Stock Purchase Plan, date of implementation	Mar. 10, 2017
Employee Stock Purchase Plan, date of first purchase	Sep. 13, 2017