Employee Benefits Tables (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure
Total net periodic postretirement benefit cost relating to Salaried VEBA			$ 4.6	$ 3.4
Gain on removal of Union VEBA net assets	$ 0.0	$ 0.5	0.0	(0.8)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Defined Contribution Plan, Cost	1.7	1.4	7.3	7.5
Deferred Compensation Arrangement with Individual, Compensation Expense	0.2	0.5	1.2	1.4
Multiemployer Plan, Contributions by Employer	1.2	1.1	3.5	3.4
Total other employee benefit plans	4.5	4.7	16.5	14.9
Postretirement Health Coverage | Salaried VEBA
Defined Benefit Plan Disclosure
Total net periodic postretirement benefit cost relating to Salaried VEBA	1.4	1.2	4.5	3.4
Salaried VEBA:
Interest cost	0.7	0.8	2.1	2.3
Expected return on plan assets	(0.8)	(1.0)	(2.2)	(3.1)
Amortization of prior service cost	1.3	1.2	4.0	3.6
Amortization of net actuarial loss	0.2	0.2	0.6	0.6
Postretirement Health Coverage | Union VEBA
Defined Benefit Plan Disclosure
Gain on removal of Union VEBA net assets	$ 0.0	$ (0.5)	$ 0.0	$ 0.8