Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) $ in Millions	Dec. 31, 2021 USD ($) numberOfTrust	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of VEBA trusts | numberOfTrust	2
Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	$ 0.3	$ 1.2	$ 4.1	$ 6.2
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	26.1	33.7	$ 34.2
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	12.2	19.9
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	13.9	13.8
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	$ 0.0	0.0
Defined Benefit Plan, Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Asset allocation strategy for the Non-Represented trust	30.00%
Fixed Income Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Asset allocation strategy for the Non-Represented trust	70.00%
U.S. equity securities | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	$ 5.7	10.5
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	5.7	10.5
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
International equity securities | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	2.0	2.0
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	2.0	2.0
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
Corporate bonds | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	10.1	9.6
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	10.1	9.6
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
U.S. government securities | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	6.9	5.2
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	3.1	1.0
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	3.8	4.2
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
Cash funds | Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	1.4	6.4
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	1.4	6.4
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	0.0	0.0
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	$ 0.0	$ 0.0