Commitments and Contingencies - Lease agreements (Details) - USD ($)		12 Months Ended
	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments and contingencies
Rent expense		$ 909,000	$ 651,000	$ 575,000
Lease agreements | 20th floor of 420 Lexington Avenue, New York
Commitments and contingencies
Extension to existing lease term	3 years
Monthly rate of lease		62,000
Lease agreements | Small laboratory and several offices at Bucks County Biotechnology Center in Doylestown, Pennsylvania
Commitments and contingencies
Monthly rate of lease		3,700
Lease agreements | Wayne, Pennsylvania
Commitments and contingencies
Monthly rate of lease		$ 10,000