Business Segments - Revenue by timing of transfer of goods or services (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenue	$ 3,010	$ 5,401	$ 9,754	$ 12,073
Goods or services transferred at a point in time
Segment Reporting Information [Line Items]
Total revenue	1,061	2,803	3,189	5,342
Goods or services transferred over time
Segment Reporting Information [Line Items]
Total revenue	$ 1,949	$ 2,598	$ 6,565	$ 6,731