Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2013	Dec. 31, 2016
Commitments And Contingencies Disclosure [Abstract]
Incentive awarded to the company by the State of Connecticut, total		$ 9.9	$ 11.5
Incentive awarded to the company by the State of Connecticut, non-interest bearing loans		7.5	8.0
Incentive awarded to the company by the State of Connecticut, grants		$ 2.4	$ 3.5
Non-interest bearing incentive loan term		10 years
Forgiveness awarded to the company by the state of Connecticut		$ 7.0
Incentive awarded to the company by the State of Connecticut, revenue recognized	$ 0.4	0.9
Incentive awarded to the company by the State of Connecticut, deferred revenue	$ 4.8	$ 4.8		$ 4.8