Revenue recognition - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholders' deficit	$ 91,248	$ 146,129	$ 136,937
Membership life, estimated duration	2 years
Membership period	12 months
Deferred income taxes	$ 406,699	$ 411,067	407,782
Total adjustments | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total stockholders' deficit	(10,853)		9,192
Deferred revenue			12,477
Deferred income taxes	$ (3,285)		$ 3,285