Summary of Significant Accounting Policies - Recent accounting pronouncements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Recent accounting pronouncements
Other nonoperating income/expense	$ (4,000)	$ 85,000	$ 57,000
Adjustment | Accounting Standards Update 2017-07
Recent accounting pronouncements
Cost of revenue and selling, general, and administrative expense		33,000	34,000
Other nonoperating income/expense		$ 33,000	$ 34,000
Adjustment | ASU 2016-01 | Accumulated Deficit
Recent accounting pronouncements
Reclassification on adoption of ASU 2016-01	73,000			$ 73,000
Adjustment | ASU 2016-01 | Accumulated Other Comprehensive Loss
Recent accounting pronouncements
Reclassification on adoption of ASU 2016-01	$ (73,000)			$ (73,000)