Retirement plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Comprehensive Income (Loss) [Line Items]
Current year actuarial loss	$ (21,337)	$ (40,069)	$ (428,496)
Pension and other postretirement benefit items	(13,269)	$ 71,293	$ (396,368)
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year actuarial loss	(23,203)
Amortization of previously deferred actuarial loss	7,615
Amortization of previously deferred prior service costs	670
Adjustment due to spin-off of publishing businesses	(4,386)
Pension and other postretirement benefit items	$ (19,304)