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Retirement Plans - Reconciliation of Balances of Pension Benefit Obligation and Fair Value of Plan Assets (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Change in pension benefit obligation:      
Benefit obligation at beginning of year $ 274,400,000 $ 253,200,000  
Service cost 800,000 1,600,000 $ 800,000
Interest cost 8,000,000.0 9,800,000 9,300,000
Net actuarial loss 22,600,000 24,700,000  
Benefits paid (15,200,000) (14,900,000)  
Benefit obligation at end of year 290,600,000 274,400,000 253,200,000
Change in plan assets:      
Fair value of plan assets at beginning of year 243,700,000 220,000,000.0  
Actual return on plan assets 34,300,000 38,600,000  
Contributions by the company 0 30,000,000.0 0
Benefits paid (15,200,000) (14,900,000)  
Fair value of plan assets at end of year 262,800,000 243,700,000 $ 220,000,000.0
Funded status of the plan (underfunded) (27,800,000) (30,700,000)  
Amounts recognized in the consolidated balance sheet consist of:      
Noncurrent liabilities - accrued benefit liability (27,800,000) (30,700,000)  
Accumulated other comprehensive loss - net actuarial loss 68,300,000 69,800,000  
Net amount included in retained earnings $ 40,500,000 $ 39,100,000