Collaborative Arrangement - Cystic Fibrosis Foundation Therapeutics Incorporated (Details) - USD ($)	1 Months Ended	3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 30, 2011	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2013	Dec. 31, 2012
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenues		$ 675,000			$ 611,000	$ 749,000	$ 1,453,000
Cystic Fibrosis Foundation Therapeutics Incorporated
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative funding	$ 75,000,000
Number of years over which funding will be made (in years)	5 years
Collaborative revenues						$ 0	$ 0
Milestone Revenues			$ 13,900,000	$ 13,900,000				$ 9,300,000	$ 9,300,000