Income taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards		$ 34,545	$ 23,838
Outside basis differences for U.S. partnerships		9,558	14,306
Stock options		2,396	4,100
Deferred revenue		640	748
Deferred compensation		120	249
State taxes		80	78
Other		1,525	1,282
Valuation allowance		(33,810)	(34,990)	$ (36,331)	$ (19,548)
Net deferred tax assets		15,054	9,611
Deferred tax liabilities:
Property and equipment		(7,318)	(6,983)
Convertible Notes		(5,470)	0
Intangible assets		(3,339)	(3,632)
Net deferred tax liabilities		(16,127)	(10,615)
Net deferred taxes		$ (1,073)	$ (1,004)
Corporate federal tax rate	21.00%	21.00%	34.00%	34.00%
Taxable income which can be offset by operating losses carryforward (as a percent)	80.00%
Executive compensation limit for covered employees	$ 1,000
Permitted bonus depreciation		100.00%
Maximum interest expense deductible, as percent to taxable income	30.00%
Income tax benefit resulting from reduction of corporate tax rate			$ 1,274
Income tax benefit from indefinite carryforward of NOLs expected to recover deferred tax liabilities			$ 1,766