Other Retirement Plans (Schedule of Covered Employees and Contributions, Multiemployer Plans) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) employee	Sep. 30, 2017 USD ($) employee	Sep. 30, 2018 USD ($) employee	Sep. 30, 2017 USD ($) employee
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	$ 35.9	$ 39.4	$ 71.1	$ 76.1
Multiemployer Plans, Pension [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	35.0	36.7	66.0	68.1
Multiemployer Plans, Other Multiemployer [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	$ 0.9	$ 2.7	$ 5.1	$ 8.0
Workforce Subject to Collective Bargaining Arrangements [Member] | Unionized Employees [Member] | Low [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, covered employees (in number of employees) | employee	6,183	3,669	2,018	550
Workforce Subject to Collective Bargaining Arrangements [Member] | Unionized Employees [Member] | High [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, covered employees (in number of employees) | employee	6,336	7,057	6,336	7,057