Employee Benefits Tables (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Defined Contribution Plan, Cost	$ 3.9	$ 3.9
Deferred Compensation Arrangement with Individual, Compensation Expense	(0.4)	0.7
Multiemployer Plan, Contributions by Employer	1.3	1.2
Total other employee benefit plans	6.0	7.5
Postretirement Health Coverage | Salaried VEBA
Defined Benefit Plan Disclosure
Total net periodic postretirement benefit cost relating to Salaried VEBA	1.2	1.7
Salaried VEBA:
Interest cost	0.6	0.8
Expected return on plan assets	(0.7)	(0.6)
Amortization of prior service cost	1.2	1.4
Amortization of net actuarial loss	$ 0.1	$ 0.1