Discontinued operations - Income Statement Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues			$ 2,877,154
Operating expenses			2,466,588
Income (loss) from discontinued operations, before income taxes			410,422
Provision for income taxes			117,342
Income (loss) from discontinued operations, net of tax	$ (232,916)	$ 178,879	293,080
Net loss (income) attributable to noncontrolling interests from discontinued operations			(63,164)
Digital
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	647,021	1,340,489	1,286,123
Operating expenses	923,683	1,071,028	1,003,259
Income (loss) from discontinued operations, before income taxes	(277,741)	256,863	277,270
Provision for income taxes	44,826	77,984	86,254
Income (loss) from discontinued operations, net of tax	(232,916)	178,879	191,016
Net loss (income) attributable to noncontrolling interests from discontinued operations	$ 58,698	$ (51,302)	(63,164)
Publishing
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues			1,400,006
Operating expenses			1,262,583
Income (loss) from discontinued operations, before income taxes			169,220
Provision for income taxes			43,735
Income (loss) from discontinued operations, net of tax			125,485
Net loss (income) attributable to noncontrolling interests from discontinued operations			0
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues			191,025
Operating expenses			200,746
Income (loss) from discontinued operations, before income taxes			(36,068)
Provision for income taxes			(12,647)
Income (loss) from discontinued operations, net of tax			(23,421)
Net loss (income) attributable to noncontrolling interests from discontinued operations			$ 0