Rate Matters Rate Matters - NSP-Wisconsin (Details) - Subsequent Event - NSP-Wisconsin $ in Millions	Apr. 01, 2016 USD ($)
PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case - Electric Rates 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 17.4
Public Utilities, Requested Rate Increase (Decrease), Percentage	2.40%
Public Utilities, Requested Rate Base, Amount	$ 1,188.0
Public utilities, Requested Increase Related to Rate Base Investments	11.0
Public Utilities, Requested Increase Related to Generation and Transmission Expenses	6.8	[1]
Public Utilities, Requested Increase Related to Fuel and Purchased Power Expenses	11.0
Public Utilities, Total Requested Rate Increase Excluding Refunds	28.8
Public Utilities, Requested Decrease Related to Fuel Refunds	(9.5)
Public Utilities, Requested Decrease Related to Settlement Refund	(1.9)
PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case - Gas Rates 2017 [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 4.8
Public Utilities, Requested Rate Increase (Decrease), Percentage	3.90%
Public Utilities, Increase Related Environmental Remediation Expenses	$ 4.8
PSCW Proceeding - Wisconsin 2017 Electric and Gas Rate Case [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.00%
Public Utilities, Percentage of Excess Earnings to be Refunded due to Earnings Cap	100.00%

[1] Includes Interchange Agreement billings. The Interchange Agreement is a Federal Energy Regulatory Commission (FERC) tariff under which NSP-Wisconsin and its affiliate, NSP-Minnesota, own and operate a single integrated electric generation and transmission system and both companies pay a pro-rata share of system capital and operating costs. For financial reporting purposes, these expenses are included in operating and maintenance expenses.