Retirement savings plan (Narrative) (Details) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Retirement Savings Plan Narrative Details
Contribution to 401 (k) plan	$ 214,000	$ 108,000	$ 0

The amount of payment into the fund established for purposes of making future disbursements to the individual in accordance with the terms and understanding of the deferred compensation arrangement.

No definition available.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration