Income taxes - Narrative (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Capital loss carryforwards	$ 488,800
Deferred tax assets valuation allowance	136,418	$ 209,939
Increase (decrease) in valuation allowance	(73,500)
Unrecognized tax benefits that, if recognized, would impact effective tax rate	10,700	10,800
Recognized interest and the release of penalty reserves	300	700	$ 400
Accrued interest and penalties payable related to unrecognized tax benefits	1,600	$ 1,500
Estimated decrease in gross unrecognized tax positions within the next 12 months, maximum	3,900
State
Income Taxes [Line Items]
Tax credit carryforward	$ 22,700
Tax years subject to examination	Tax years before 2013 remain subject to examination by certain states due to ongoing audits.
Federal
Income Taxes [Line Items]
Tax years subject to examination	The 2013 through 2017 tax years remain subject to examination by the Internal Revenue Service and state authorities.
Tax Year 2020
Income Taxes [Line Items]
Capital loss carryforwards subject to expiration	$ 361,500
Capital Loss Deferred Tax Assets
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	40,200
CareerBuilder, LLC Deferred Tax Assets
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	(42,900)
Tax Reform Deferred Tax Assets
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	$ (70,800)