Business (Details) (USD $)	0 Months Ended		3 Months Ended		87 Months Ended
	Feb. 18, 2014	Sep. 30, 2013	Mar. 31, 2014 item	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013
Business
Number of business segments			1
Net loss			$ 20,682,000	$ 4,618,000	$ 172,457,000
Accumulated deficit during the development stage			203,732,000		203,732,000	183,050,000
Common stock issued in IPO (in shares)	1,900,000	8,740,000
Public offering price (in dollars per share)		$ 22.00
Shares issued on exercise of over-allotment option by underwriters		1,140,000
Net proceeds from IPO after deducting underwriting discounts and commissions and other offering expenses		175,600,000			175,555,000
Shares of common stock offered in follow on offering	2,628,571
Offering price (in dollars per share)	$ 31.50
Shares sold by selling stockholders in follow on offering	728,571
Shares sold by selling stockholders upon full exercise by underwriters of their option to purchase additional shares	342,857
Net proceeds, after deducting underwriters' discounts and commissions and other offering expense	55,400,000		55,409,000		55,409,000
Cash, cash equivalents, and marketable securities			290,800,000		290,800,000
Potential future funding under the royalty agreement with Novo A/S			$ 41,700,000