Income taxes - Narrative (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
Operating Loss Carryforwards [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	$ (36,563)	$ (28,545)	$ (115,692)	$ (117,413)
Effective Income Tax Rate Reconciliation, Percent	(1.90%)	(7.00%)	1.80%	(20.70%)
Current Foreign Tax Expense (Benefit)	$ 500		$ 900
Income Tax Effects Allocated Directly to Equity, Other	1,500		2,600
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions			10,900
Other Tax Expense (Benefit)			300
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	1,300		9,400
Proceeds from Income Tax Refunds			32,900
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			26,000
Income tax benefit, net valuation allowance			2,600
Income Tax Expense (Benefit)	706	$ 1,991	(2,076)	$ 24,273
Unrecognized tax benefits	33,700		33,700		$ 58,584
Unrecognized tax benefits that would impact effective tax rate	$ 16,900		16,900
Restructuring Charges [Member]
Operating Loss Carryforwards [Line Items]
Other Tax Expense (Benefit)			$ 4,400