Retirement plans - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions expected to be made during next fiscal year	$ 11,600,000
401(k) employee maximum matching contribution	50.00%
401(k) employer matching contribution	100.00%
Contributions per employee subject to employer match	4.00%
Compensation expense related to 401(k) contributions	$ 13,300,000	$ 14,400,000	$ 13,500,000
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]
Acquisition of KFMB	$ 25,966,000	0
Accumulated other comprehensive loss	184,498,000	177,471,000
Accumulated benefit obligation	554,768,000	$ 614,079,000
Contributions expected to be made during next fiscal year	3,800,000
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	6,000,000
Prior service credit estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	100,000
Expected pension expense, net of contributions	$ 3,500,000
Actual rate of return on plan assets (as a percent)	(5.60%)	(20.30%)	(7.40%)
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions expected to be made during next fiscal year	$ 7,800,000
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost	$ 700,000
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	$ 2,400,000	$ 1,800,000
KFMB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net pension obligation	$ 7,300,000