Commitments and Contingencies, Environmental Contingencies - Site Contingencies (Details) $ in Millions	1 Months Ended		12 Months Ended
	Feb. 28, 2017	Dec. 31, 2016 USD ($) Site	Dec. 31, 2016 USD ($) Site	Dec. 31, 2015 USD ($)
Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Number of Properties Not Owned Included in Superfund Site | Site		2	2
Accrual for Environmental Loss Contingencies, Gross		$ 64.3	$ 64.3	$ 94.4
Approved Amortization Period for Recovery of Remediation Costs in Natural Gas Rates			10 years
Carrying Cost Percentage to be Applied to Unamortized Regulatory Asset			3.00%
Ashland MGP Site Phase I Project Area [Member] | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross		72.4	$ 72.4
Estimated Amount Spent on Cleanup		56.7	56.7
Fargo MGP Site | NSP-Minnesota
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross		11.3	$ 11.3	2.7
Percentage of Response Costs Allocable to the Minnesota Jurisdiction			12.00%
Other MGP Sites
Manufactured Gas Plant (MGP) Site [Abstract]
Accrual for Environmental Loss Contingencies, Gross		$ 2.0	$ 2.0	$ 2.1
Number of Identified MGP Sites Under Current Investigation and/or Remediation		7	7
PSCW Proceeding - Electric and Gas Rate Case 2016 - Gas Rates 2016 [Member] | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved Annual Recovery, For 2016, Collected Through Base Rates			$ 7.6
PSCW Proceeding - Gas Rate Case 2017 - Gas Rates 2017 | Ashland MGP Site | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Public Utilities, Approved Annual Recovery Collected Through Base Rates		$ 12.4
Subsequent Event | Ashland MGP Site Phase II Project Area [Member] | NSP-Wisconsin
Manufactured Gas Plant (MGP) Site [Abstract]
Length of Public Comment Period that Lapsed	30 days