Income Taxes (Details 2) - USD ($) $ in Thousands		1 Months Ended			7 Months Ended	12 Months Ended
	May. 19, 2014	Mar. 31, 2015	Sep. 30, 2014	May. 31, 2014	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income taxes
Proceeds from the Novo Agreement						$ 83,333	$ 41,667
Novartis Pharma AG
Income taxes
Amount received				$ 200,000
Patient enrollment-based milestone revenue achieved		$ 50,000	$ 50,000		$ 100,000
Novartis Pharma AG | Licensing and Commercialization Agreement
Income taxes
Amount received	$ 200,000
Novartis Pharma AG | Achievement of specified patient enrollment milestones | Licensing and Commercialization Agreement
Income taxes
Patient enrollment-based milestone revenue achieved		$ 50,000	$ 50,000		$ 100,000