Summary of significant accounting policies - Recent accounting pronouncements (Details) - USD ($)	6 Months Ended
	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Lease, Cost [Abstract]
Operating lease right-of-use assets	$ 3,274,000
Operating lease liabilities	$ 4,214,000
Deferred rent			$ 989,000
ASU 2016-02
Lease, Cost [Abstract]
Operating lease liabilities		$ 4,300,000
ASU 2018-11
Lease, Cost [Abstract]
Lease, Practical Expedients, Package [true false]	true
Lease, Practical Expedient, Land Easement [true false]	true
Operating lease right-of-use assets		3,400,000
Operating lease liabilities		4,300,000
Deferred rent		$ 900,000