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Retirement Plans - Defined Contribution Retirement Plans (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Expense associated with contributions to defined benefit contribution retirement plans $ 130 $ 115 $ 150
Multiemployer plan, employer contribution, cost $ 38 $ 32 $ 30