Nature of Business - Additional Information (Detail) (USD $)	12 Months Ended		12 Months Ended	0 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Minimum [Member] Diseases	Jul. 29, 2013 IPO [Member]	Jul. 29, 2013 Private Placement [Member]	Jul. 29, 2013 Celgene [Member]
Nature Of Business [Line Items]
Number of rare genetic disease under entity therapeutic group			600
Accumulated deficit	$ (113,444,000)	$ (74,037,000)
Common stock shares purchased				6,772,221	708,333
Public offering price				$ 18.00	$ 18.00
Number of shares of common stock issued upon the exercise in full by the underwriters of their option to purchase additional shares at the public offering price				883,333
Net proceeds from the IPO	113,367,000			111,000,000
Proceeds from issuance of IPO	110,980,000					12,800,000
Cash, cash equivalents and marketable securities	$ 193,900,000