Common stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		36 Months Ended	1 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2014 Common stock	Mar. 31, 2013 Issuance under private placement Common stock
Common stock
Common stock issued (in shares)				5,518,630
Common stock issued in a private placement (in shares)					3,888,880
Common stock issue price (in dollars per share)				$ 10.00	$ 9.00
Number of shares represents partial exercise of overallotment option granted to underwriters				518,630
Net proceeds from issuance of common stock	$ 51,661	$ 32,652	$ 88,116	$ 51,700	$ 32,700