Employee Benefits - Summary of Total Expense Related to All Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Defined contribution plan, cost	$ 1.7	$ 1.7	$ 5.6	$ 5.6
Deferred compensation arrangement with individual, compensation expense	0.8	0.3	0.4	1.0
Multiemployer plan, contributions by employer	1.2	1.2	2.5	2.4
Total other employee benefit plans	4.9	4.8	10.9	12.3
Postretirement Health Coverage | Salaried VEBA
Defined Benefit Plan Disclosure
Total net periodic postretirement benefit cost relating to Salaried VEBA	$ 1.2	$ 1.6	$ 2.4	$ 3.3