Retirement plans - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Spin-off of Publishing businesses	$ 1,513,881,000	$ 45,128,000	$ 1,194,271,000
Deferred tax assets	220,232,000	344,487,000
Contributions expected to be made during next fiscal year	41,400,000
Expected pension expense, net of contributions	$ 2,400,000
401(k) employee maximum matching contribution	50.00%
401(k) employer matching contribution	100.00%
Contributions per employee subject to employer match	5.00%
Compensation expense related to 401(k) contributions	$ 14,400,000	13,500,000	$ 17,100,000
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer's contributions as a percent of total contribution	5.00%
Plans in red zone | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multi-employer plans funded percentage (less than 65% of red zone, less than 80% of orange zone, and at least 80% of green zone)	Less than 65 percent
Plans in yellow zone | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multi-employer plans funded percentage (less than 65% of red zone, less than 80% of orange zone, and at least 80% of green zone)	Between 65 and less than 80 percent
Plans in green zone | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multi-employer plans funded percentage (less than 65% of red zone, less than 80% of orange zone, and at least 80% of green zone)	At least 80 percent
Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Adjustment due to spin-off of publishing businesses	$ 0	13,639,000
Accumulated other comprehensive loss	177,471,000	207,478,000
Accumulated benefit obligation	614,079,000	$ 601,430,000
Contributions expected to be made during next fiscal year	10,700,000
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	5,100,000
Prior service credit estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	$ 200,000
Actual rate of return on plan assets (as a percent)	20.30%	7.40%	1.00%
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions expected to be made during next fiscal year	$ 30,700,000
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	$ (6,300,000)
Multiemployer Plans, Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected funding deficiency, period	6 years
Expenses incurred for multi-employer withdrawal liabilities	$ 0	$ 0
AFTRA Plan | Multiemployer Plans, Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
EIN Number	136414972
Multiemployer Plan Number	001
Contributions	$ 2,400,000	$ 1,800,000	$ 1,100,000
Adjustments to Postretirement Benefits Transferred, Spinoff Transaction | Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Adjustment due to spin-off of publishing businesses	13,600,000
Spin-off of Publishing businesses	7,700,000
Accumulated other comprehensive loss	2,600,000
Deferred tax assets	6,400,000
Adjustments to Postretirement Benefits Transferred, Spinoff Transaction | Postretirement Medical and Life Insurance
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Adjustment due to spin-off of publishing businesses	$ 3,100,000