Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - Jun. 30, 2015 - NSP-Wisconsin - Bonds [Member] - Series Due June 15, 2024 [Member] - USD ($) $ in Millions	Total
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 100
Debt Instrument, Interest Rate, Stated Percentage	3.30%