Retirement Plans & Other Postretirement Benefits - Multiemployer Contributions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	$ 42.1	$ 35.0	$ 32.7
Central Pension Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	9.3	7.4	7.2
Indiana Laborers Pension Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	5.0	4.4	4.1
Pipeline Industry Benefit Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	4.9	3.0	4.0
Laborers District Fund of Ohio
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	3.3	2.0	1.5
Ohio Operating Engineers Pension Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	2.8	2.1	2.2
Operating Eng. Local 324 Fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	2.5	1.6	1.6
Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Multiemployer contributions	$ 14.3	$ 14.5	$ 12.1