Revenue recognition - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of transition to ASC 606	$ 382,789	$ 146,129	$ 136,937	$ 214,755	$ 1,080
Equipment deposits	$ 7,908		6,498
Deferred revenue expected recognition period	12 months
Deferred revenue	$ 49,862		40,000
Deferred income taxes	414,841	$ 411,067	407,782
Total adjustments | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Cumulative effect of transition to ASC 606	(14,033)		9,192
Deferred revenue			12,477
Deferred income taxes	$ (3,285)		$ 3,285