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, 2017	Dec. 31, 2016	Dec. 31, 2015
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	$ 20,373	$ (21,337)	$ (40,069)
Curtailment gain	0	0	79,184
Pension and other postretirement benefit items	29,210	(13,269)	$ 71,293
Retirement Plans
Comprehensive Income (Loss) [Line Items]
Current year net actuarial gain (loss)	16,272	(23,203)
Amortization of previously deferred actuarial loss	8,357	7,615
Amortization of previously deferred prior service costs	635	670
Curtailment gain	4,716	0
Prior service cost recognized in curtailment	26	0
Adjustment due to spin-off of publishing businesses	0	(4,386)
Pension and other postretirement benefit items	$ 30,006	$ (19,304)