Revenues (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	May 26, 2020
Series Due May 1, 2051 [Domain] | Bonds [Member]
Disaggregation of Revenue [Line Items]
Debt Instrument, Face Amount					$ 100,000,000
Series Due May 1, 2051 [Domain] | Bonds [Member] | NSP-Wisconsin [Member]
Disaggregation of Revenue [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					305.00%
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 218,500,000	$ 228,900,000	$ 475,500,000	$ 505,100,000
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	199,300,000	207,900,000	411,100,000	422,900,000
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	19,000,000.0	20,900,000	64,200,000	82,000,000.0
All Other | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	200,000	100,000	200,000	200,000
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	215,100,000	225,300,000	468,300,000	497,900,000
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	196,400,000	204,900,000	404,800,000	416,900,000
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	18,500,000	20,300,000	63,300,000	80,800,000
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	200,000	100,000	200,000	200,000
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	172,400,000	180,800,000	383,400,000	407,700,000
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	70,200,000	64,900,000	159,700,000	165,900,000
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	100,000,000.0	113,700,000	219,900,000	238,000,000.0
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,200,000	2,200,000	3,800,000	3,800,000
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	154,800,000	161,300,000	321,800,000	328,700,000
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	59,600,000	54,100,000	124,600,000	121,500,000
Retail | Regulated Electric | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	93,200,000	105,100,000	193,600,000	203,600,000
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,000,000.0	2,100,000	3,600,000	3,600,000
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	17,400,000	19,400,000	61,400,000	78,800,000
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10,600,000	10,800,000	35,100,000	44,400,000
Retail | Regulated Natural Gas | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6,800,000	8,600,000	26,300,000	34,400,000
Retail | Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Retail | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	200,000	100,000	200,000	200,000
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Retail | All Other | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	200,000	100,000	200,000	200,000
Interchange [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41,500,000	43,500,000	82,400,000	87,600,000
Interchange [Member] | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41,500,000	43,500,000	82,400,000	87,600,000
Interchange [Member] | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Interchange [Member] | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,200,000	1,000,000.0	2,500,000	2,600,000
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	100,000	100,000	600,000	600,000
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,100,000	900,000	1,900,000	2,000,000.0
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0	0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3,400,000	3,600,000	7,200,000	7,200,000
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	2,900,000	3,000,000.0	6,300,000	6,000,000.0
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	500,000	600,000	900,000	1,200,000
Alternative revenue and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0	$ 0	$ 0	$ 0