Retirement plans - Reconciliation of Benefit Obligations, Plan Assets and Funded Status of Company-Sponsored Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligations
Benefit obligations at end of year	$ 614,111
Change in plan assets
Fair value of plan assets at end of year	439,149
Funded status at end of year	(174,962)
Amounts recognized in Consolidated Balance Sheets
Accrued benefit cost—noncurrent	(144,220)	$ (187,290)
Retirement Plans
Change in benefit obligations
Benefit obligations at beginning of year	606,413	586,624
Service cost	872	816	$ 920
Interest cost	23,985	26,111	23,800
Actuarial loss	26,700	17,755
Gross benefits paid	(39,143)	(38,532)
Adjustment due to spin-off of publishing businesses	0	13,639
Curtailment gain	(4,716)	0
Benefit obligations at end of year	614,111	606,413	586,624
Change in plan assets
Fair value of plan assets at beginning of year	388,168	400,193
Actual return on plan assets	69,295	21,316
Employer contributions	20,829	5,191
Gross benefits paid	(39,143)	(38,532)
Fair value of plan assets at end of year	439,149	388,168	$ 400,193
Funded status at end of year	(174,962)	(218,245)
Amounts recognized in Consolidated Balance Sheets
Accrued benefit cost—current	(30,742)	(30,955)
Accrued benefit cost—noncurrent	$ (144,220)	$ (187,290)