Retirement Benefits (Details 2) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan curtailments		$ 0.0	$ (6.7)
Plan amendments		$ 0.0	$ 0.0
Discount rate	[1]	4.45%	4.65%	4.25%
Expected long-term rate of return on assets		7.75%	8.00%	8.00%
Rate of increase in future compensation levels			1.00%
Pension Benefits [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of increase in future compensation levels		0.50%	3.00%	3.00%
Rate of increase in future compensation levels		0.50%
Pension Benefits [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of increase in future compensation levels		1.00%	3.50%	3.50%
Rate of increase in future compensation levels		1.00%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan curtailments		$ 0.0	$ 0.0
Plan amendments		$ 0.0	$ (22.5)
Discount rate	[1]	4.35%		4.10%
Rate of increase in future compensation levels		0.00%	0.00%	0.00%
Expected long-term rate of return on assets		4.00%	4.00%	4.00%
Rate of increase in future compensation levels		0.00%	0.00%
Other Postretirement Benefits [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	[1]		4.05%
Other Postretirement Benefits [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	[1]		4.50%

[1] Other postretirement benefits expense for 2016 was initially measured at a 4.50% discount rate. A portion of the obligation was remeasured using a 4.05% discount rate as of March 1, 2016, following a new USW labor agreement.