Commitments and contingencies (Letters of Credit) (Details) - Letter of Credit - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 0	$ 869,000
Proceeds from Letters	0	0	$ 0
Interest paid	$ 0	$ 0	$ 0