Other Retirement Plans (Schedule of Covered Employees and Contributions, Multiemployer Plans) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) employee	Sep. 30, 2016 USD ($) employee	Sep. 30, 2017 USD ($) employee	Sep. 30, 2016 USD ($) employee
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	$ 39.4	$ 29.3	$ 76.1	$ 51.0
Multiemployer Plans, Pension [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	36.7	26.4	68.1	43.4
Multiemployer Plans, Other Multiemployer [Member]
Covered Employees and Contributions, Multiemployer Plans [Line Items]
Multiemployer plans, contributions (in dollars)	$ 2.7	$ 2.9	$ 8.0	$ 7.6
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	3,669	4,170	550	1,112
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	7,057	4,910	7,057	4,910