Other Retirement Plans (Schedule of Covered Employees and Contributions, Multiemployer Plans) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) numberOfEmployees	Dec. 31, 2019 USD ($) numberOfEmployees	Dec. 31, 2018 USD ($) numberOfEmployees
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer pension plans, contributions (in dollars)	$ 37.5	$ 72.6	$ 99.7
Low [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, covered employees (in numberof employees) | numberOfEmployees	1,119	1,119	1,626
High [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, covered employees (in numberof employees) | numberOfEmployees	2,412	5,349	6,336
Pension Plans [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer pension plans, contributions (in dollars)	$ 30.0	$ 66.9	$ 92.4
Other Multiemployer Plans [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer pension plans, contributions (in dollars)	$ 7.5	$ 5.7	$ 7.3