Other balance sheet items - Schedule of Other Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization	$ (1,560)		$ (1,560)		$ (5,808)
Indefinite-lived Intangible Assets [Line Items]
Gross carrying value	6,345		6,345		7,895
Net carrying value	4,785		4,785		2,087
Amortization expense for intangible assets	600	$ 500	300	$ 300
Licenses
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets	858		858		981
Strategic alliance
Finite-Lived Intangible Assets [Line Items]
Gross carrying value, Finite-lived intangible assets	0		0		3,616
Accumulated amortization	0		0		(3,616)
Net carrying value, Finite-lived intangible assets	0		0		0
Enterprise resource planning system
Finite-Lived Intangible Assets [Line Items]
Gross carrying value, Finite-lived intangible assets	1,050		1,050		0
Net carrying value, Finite-lived intangible assets	1,050		$ 1,050		0
Estimated useful life			7 years
Management contract
Finite-Lived Intangible Assets [Line Items]
Gross carrying value, Finite-lived intangible assets	1,900		$ 1,900		0
Net carrying value, Finite-lived intangible assets	1,900		1,900		0
Other
Finite-Lived Intangible Assets [Line Items]
Gross carrying value, Finite-lived intangible assets	2,537		2,537		3,298
Accumulated amortization	(1,560)		(1,560)		(2,192)
Net carrying value, Finite-lived intangible assets	$ 977		$ 977		$ 1,106