Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	$ 430.4	$ 0.3	$ 1.2	$ 4.1
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	17.4	26.1	$ 33.7
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	7.3	12.2
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	13.9
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	$ 3.7	5.7
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	3.7	5.7
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	1.1	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	1.1	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	7.1	10.1
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	7.1	10.1
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	5.0	6.9
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	2.0	3.1
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	3.0	3.8
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	0.5	1.4
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	0.5	1.4
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