Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense		$ 1,034	$ 1,281	$ 885
GlaxoSmithKline LLC [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 700	430	703	443
GlaxoSmithKline LLC [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense	700	7	5	10
GlaxoSmithKline LLC [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	700	597	573	432
License Fees [Member] | The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense		574	866	716
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Sublicense Fees [Member]
Other Commitments [Line Items]
Total related party transaction expense	400	430	703	443
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Royalties on Sales of Reagents [Member]
Other Commitments [Line Items]
Total related party transaction expense	400	11	9	17
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Maintenance of Licensed Patents [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 400	$ 133	$ 154	$ 256