Commitments and contingencies (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Operating lease:
Deferred rent balance	$ 1,172,000		$ 1,172,000		$ 1,189,000
Rent expense, share of operating expenses	60,000	$ 59,000	119,000	$ 126,000
Rent expense, excluding share of operating expenses	$ 218,000	$ 229,000	$ 436,000	$ 455,000