Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Change in Plan Assets [Roll Forward]
Non-Current Liabilities		$ (179.7)	$ (179.5)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net Loss	[1]	9.2	12.7
Change in Benefit Obligation [Roll Forward]
Obligation, Beginning of Year		148.2	167.6
Service Cost		3.0	3.6	$ 3.3
Interest Cost	[2]	4.4	4.4	5.0
Actuarial (Gain) Loss	[3]	(38.7)	(19.7)
Benefits Paid		(9.2)	(10.6)
Participant Contributions		2.7	2.9
Obligation, End of Year		110.4	148.2	167.6
Change in Plan Assets [Roll Forward]
Fair Value, Beginning of Year		201.8	186.0
Actual Return on Plan Assets		(33.0)	22.8
Employer Contribution		0.3	0.7
Defined Benefit Plan Contributions by Plan Participants, Excluding Key Employees		2.7	2.9
Benefits Paid		(9.2)	(10.6)
Fair Value, End of Year		162.6	201.8	186.0
Funded Status, End of Year		52.2	53.6
Non-Current Assets		58.8	64.8
Current Liabilities		(0.2)	(1.4)
Non-Current Liabilities		(6.4)	(9.8)
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net Loss		(260.2)	(260.2)
Accumulated Benefit Obligation		724.5	875.5
Change in Benefit Obligation [Roll Forward]
Obligation, Beginning of Year		911.7	965.7
Service Cost		9.3	11.0	10.7
Interest Cost	[4]	27.2	24.6	27.9
Plan Amendments		0.8	0.0
Actuarial (Gain) Loss	[5]	(160.6)	(39.0)
Benefits Paid		(58.9)	(56.5)
Participant Contributions		10.2	7.3
Plan Curtailments		0.0	(1.4)
Obligation, End of Year		739.7	911.7	965.7
Change in Plan Assets [Roll Forward]
Fair Value, Beginning of Year		745.7	759.4
Actual Return on Plan Assets		(130.5)	23.1
Employer Contribution	[6]	12.3	19.7
Benefits Paid		(58.9)	(56.5)
Fair Value, End of Year		568.6	745.7	$ 759.4
Funded Status, End of Year		(171.1)	(166.0)
Current Liabilities		(2.1)	(2.1)
Non-Current Liabilities		$ (169.0)	$ (163.9)

[1] Excludes gains, losses and contributions associated with irrevocable grantor trusts.

[2] These components of net periodic postretirement health and life cost are included in the line item “Other” under Other Income (Expense) on the Consolidated Statement of Income.

[3] Actuarial gain was primarily the result of increases in discount rates in 2022 and 2021.

[4] These components of net periodic pension cost are included in the line item “Other” under Other Income (Expense) on the Consolidated Statement of Income.

[5] Actuarial gain was primarily the result of increases in discount rates in 2022 and 2021.

[6] Includes Participant Contributions noted above, any contributions made by the Company to pension plan trusts and any direct benefit payments made under certain plans.(c)    Presented here net of amounts related to prepaid pension for SWL&P that are reclassified to Regulatory Assets on the Consolidated Balance Sheet.