Discontinued operations - Income Statement Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income (loss) from discontinued operations, net of tax	$ (7,474)	$ 102,064	$ 374,235
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		1,591,031	3,382,033
Income (loss) from discontinued operations, before income taxes		133,152	365,364
Provision for income taxes		31,088	(8,871)
Income (loss) from discontinued operations, net of tax		102,064	374,235
Publishing \| Discontinued Operations, Disposed of by Means Other than Sale, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		1,400,006	3,133,861
Income (loss) from discontinued operations, before income taxes		169,220	372,549
Provision for income taxes		43,735	(11,817)
Income (loss) from discontinued operations, net of tax		125,485	384,366
Other \| Discontinued Operations, Disposed of by Means Other than Sale, Spinoff
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		191,025	248,172
Income (loss) from discontinued operations, before income taxes		(36,068)	(7,185)
Provision for income taxes		(12,647)	2,946
Income (loss) from discontinued operations, net of tax		$ (23,421)	$ (10,131)

Amount before tax of income (loss) from a discontinued operation. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

Amount of tax expense (benefit) related to a discontinued operation. Includes, but is not limited to, tax expense (benefit) related to income (loss) from operations during the phase-out period, tax expense (benefit) related to gain (loss) on disposal, tax expense (benefit) related to gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and tax expense (benefit) related to adjustments of a prior period gain (loss) on disposal.

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- Definition

Amount after tax of income (loss) from a discontinued operation including the portion attributable to the noncontrolling interest. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

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