Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		$ 78.9	$ 20.9
Regulatory Liability, Noncurrent		370.8	400.1
Deferred Income Tax Adjustments and TCJA Refunds
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	[1]	51.0	0.2
Regulatory Liability, Noncurrent	[1]	195.9	238.3
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.0	0.0
Regulatory Liability, Noncurrent		171.2	157.7
Deferred Electric Production And Natural Gas Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		17.4	13.4
Regulatory Liability, Noncurrent		0.0	0.0
United States Department of Energy Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		9.7	6.2
Regulatory Liability, Noncurrent		0.0	0.0
Other
Regulatory Liabilities [Line Items]
Regulatory Liability, Current		0.8	1.1
Regulatory Liability, Noncurrent		$ 3.7	$ 4.1

[1] Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA. At Dec. 31, 2019 and 2018, NSP-Wisconsin’s regulatory assets not earning a return primarily included the unfunded portion of pension and retiree medical obligations. In addition, regulatory assets included $7.4 million and $7.6 million at Dec. 31, 2019 and 2018, respectively, of past expenditures not earning a return. Amounts related to funded pension obligations.