Income taxes (Details 3) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012
Reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations
Income tax benefit using U.S. federal statutory rate (as a percent)	34.00%	34.00%
State income taxes, net of federal benefit (as a percent)	5.47%	5.42%
Research and development tax credits (as a percent)	1.70%	0.00%
Permanent items (as a percent)	(3.93%)	(2.78%)
Change in the valuation allowance (as a percent)	(37.24%)	(36.64%)
Deferred tax assets:
Net operating loss carryforwards	$ 750	$ 5,527
Research and development credits	67	153
Stock-based compensation	22	142
Other	138	106
Gross deferred tax assets	977	5,928
Valuation allowance	$ (977)	$ (5,928)