Goodwill and intangible assets - Intangible assets rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Changes in intangible assets
Balance at beginning of year	$ 14,940	$ 19,152
Measurement period adjustments for acquisition of Elauwit		(48)
Reclassification of assets held for sale, net		407
Amortization expense	(4,288)	(4,571)
Balance at end of year	$ 10,652	$ 14,940