Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) numberOfTrust	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
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	$ 1.2	$ 4.1	$ 6.2	$ 11.8
Other Postretirement Benefits Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, assets	33.7	34.2	$ 15.0
Voluntary employees beneficiary association, contribution amount to Non-Represented Trust	10.7
Voluntary employees beneficiary association, contribution amount to Represented Trust	9.6
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	19.9	20.5
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	13.8	13.7
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%	50.00%
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%	50.00%
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	$ 10.5	$ 13.0
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	10.5	13.0
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	4.0
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	4.0
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	9.6	9.2
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	9.6	9.2
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	5.2	7.1
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	1.0	2.6
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	4.2	4.5
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	6.4	0.9
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	6.4	0.9
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