Business (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended		81 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012
Business
Number of business segments			1
Net loss	$ 16,132	$ 4,153	$ 30,757	$ 10,731	$ 131,387
Accumulated deficit during the development stage	162,661		162,661		162,661	126,471
Common stock issued and sold in IPO (in shares)			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			$ 175,555		$ 175,555