Basis of presentation and significant accounting policies - Other current assets (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Inventory	[1]	$ 8,063	$ 6,974
Prepaid expenses and other		6,228	7,600
Other current assets		$ 14,291	$ 14,574

[1] See Note 2.j for discussion of inventory held by the Company.