Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year		$ (130,951)	$ (778,769)	$ (494,055)
Other comprehensive income (loss) before reclassifications	$ 24,439	(32,920)
Spin-off publishing businesses	603,469	(2,642)
Other comprehensive loss before reclassifications				(312,283)
Amounts reclassified from AOCL	19,910	4,940		27,569
Balance at end of year	(130,951)	(161,573)	(130,951)	(778,769)
Retirement Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year		(116,496)	(1,172,245)	(923,595)
Other comprehensive income (loss) before reclassifications	23,094	(13,143)
Spin-off publishing businesses	1,012,745	(2,642)
Other comprehensive loss before reclassifications				(276,219)
Amounts reclassified from AOCL	19,910	4,940	19,910	27,569
Balance at end of year	(116,496)	(127,341)	(116,496)	(1,172,245)
Foreign Currency Translation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year		(20,129)	391,113	427,177
Other comprehensive income (loss) before reclassifications	(1,966)	(8,431)
Spin-off publishing businesses	(409,276)	0
Other comprehensive loss before reclassifications				(36,064)
Amounts reclassified from AOCL	0	0		0
Balance at end of year	(20,129)	(28,560)	(20,129)	391,113
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year		5,674	2,363	2,363
Other comprehensive income (loss) before reclassifications	3,311	(11,346)
Spin-off publishing businesses	0	0
Other comprehensive loss before reclassifications				0
Amounts reclassified from AOCL	0	0		0
Balance at end of year	$ 5,674	$ (5,672)	$ 5,674	$ 2,363