Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Regulated and Unregulated Operating Revenue	$ 232.1	$ 243.7	$ 228.9	$ 276.2	$ 260.6		$ 256.0		$ 231.8		$ 273.1		$ 980.9	$ 1,021.5	$ 1,005.5
Operating income	19.1	47.7	21.8	42.0	34.9	[1]	48.0	[1]	27.5	[1]	49.0	[1]	130.6	159.4	152.6
Net income	$ 16.9	$ 29.4	$ 8.6	$ 24.0	$ 20.4		$ 31.0		$ 15.2		$ 31.4		$ 78.9	$ 98.0	$ 79.4

[1] In 2018, NSP-Wisconsin implemented ASU No. 2017-07 related to net periodic benefit cost, which resulted in retrospective reclassification of pension costs from O&M expense to other income.