Summary of significant accounting policies - Recent accounting pronouncements (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Sep. 30, 2019	Dec. 31, 2018
Lease, Cost [Abstract]
Operating lease right-of-use assets		$ 3,161
Operating lease liabilities		$ 4,072
Deferred rent			$ 989
ASU 2016-02
Lease, Cost [Abstract]
Operating lease liabilities	$ 4,300
ASU 2018-11
Lease, Cost [Abstract]
Lease, practical expedients package	true
Lease, practical expedient land easement	true
Operating lease right-of-use assets	$ 3,400
Operating lease liabilities	$ 4,300
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
ASU 2018-02
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true