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Retirement plans - Narrative (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Expected pension expense, net of contributions $ 5,300,000    
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 $ 14,600,000 $ 13,300,000 $ 14,400,000
Retirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Accumulated benefit obligation 613,655,000 $ 554,768,000  
Contributions by employer 4,000,000.0    
Discretionary contributions by employer 12,000,000    
Contributions expected to be made during next fiscal year 6,700,000    
Actuarial loss estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost 6,100,000    
Prior service credit estimated to be amortized from accumulated other comprehensive loss into net periodic benefit cost $ 100,000    
Actual rate of return on plan assets (as a percent) 23.60% (5.60%) 20.30%
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 $ 0
AFTRA Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Contributions $ 2,400,000