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, 2015	Dec. 28, 2014	Dec. 29, 2013
Comprehensive Income (Loss) [Line Items]
Current year actuarial loss	$ 39,115	$ (466,482)	$ 286,778
Amortization of previously deferred actuarial loss	31,357	46,489	64,381
Amortization of previously deferred prior service costs	0	37,986	319
Pension and other postretirement benefit items	71,293	$ (396,368)	$ 342,798
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year actuarial loss	(43,821)
Amortization of previously deferred actuarial loss	6,335
Amortization of previously deferred prior service costs	673
Pension and other postretirement benefit items	$ (36,813)