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Retirement Plans - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution plan, cost recognized $ 23,641,000 $ 25,226,000 $ 23,417,000
Defined benefit plan, voluntary contribution $ 10,446,000 1,112,000  
Corridor amount 10.00%    
Net unrecognized actuarial losses expected to be recognized in 2018 $ 10,400,000,000    
Net unrecognized actuarial losses expected to be recognized in 2018, net of tax 7,700,000,000    
Net periodic benefit cost (income), non-service cost (569,000) 3,559,000 (2,590,000)
U.S.      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, planned contributions for each of next 5 years   $ 9,000,000  
Defined benefit plan, voluntary contribution     $ 10,000,000
Increase (decrease) in projected benefit obligation $ (3,063,000)    
Discount rate used to compute periodic benefit cost 3.92%    
Expected long-term rates of return on plans' assets in next fiscal year 4.70%    
U.K Plan      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate used to compute periodic benefit cost 1.71%    
Expected long-term rates of return on plans' assets in next fiscal year   2.10%