Regulatory Assets and Liabilities, Regulatory Assets (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets [Line Items]
Regulatory Asset, Current		$ 20.0	$ 26.0
Regulatory Asset, Noncurrent		364.0	366.2
Pension and Retiree Medical Obligations
Regulatory Assets [Line Items]
Regulatory Asset, Current		11.1	12.6
Regulatory Asset, Noncurrent		203.5	222.1
Excess deferred taxes - TCJA
Regulatory Assets [Line Items]
Regulatory Asset, Current		1.7	0.0
Regulatory Asset, Noncurrent		52.0	55.9
Recoverable Deferred Taxes on AFUDC Recorded in Plant
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.0	0.0
Regulatory Asset, Noncurrent		34.1	27.9
Net AROs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[1]	0.0	0.0
Regulatory Asset, Noncurrent	[1]	26.9	25.7
Losses on Reacquired Debt
Regulatory Assets [Line Items]
Regulatory Asset, Current		0.8	0.8
Regulatory Asset, Noncurrent		21.0	21.9
Conservation Programs
Regulatory Assets [Line Items]
Regulatory Asset, Current	[2]	0.6	0.7
Regulatory Asset, Noncurrent	[2]	1.1	0.6
Other
Regulatory Assets [Line Items]
Regulatory Asset, Current		5.8	11.9
Regulatory Asset, Noncurrent		$ 25.4	$ 12.1

[1] Includes amounts recorded for future recovery of AROs.

[2] Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.