Rate Matters Rate Matters (Notes)	3 Months Ended
Mar. 31, 2017
Public Utilities, General Disclosures [Abstract]
Rate Matters	Rate Matters Except to the extent noted below, the circumstances set forth in Note 10 to the consolidated financial statements included in NSP-Wisconsin’s Annual Report on Form 10-K for the year ended Dec. 31, 2016, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference. Recently Concluded Regulatory Proceedings - Michigan Public Service Commission (MPSC) Michigan 2017 Natural Gas Rate Case — In October 2016, NSP-Wisconsin filed a request with the MPSC to increase base rates for natural gas service by approximately $347 thousand annually, or 6.5 percent. The filing was based on a 2017 forecast test year, a 10.2 percent return on equity (ROE), an equity ratio of 52.56 percent and a forecasted average rate base of approximately $6.4 million. The primary driver of the requested increase is investment in natural gas distribution infrastructure, mainly in conjunction with the NSP-Wisconsin’s Distribution Integrity Management Program (DIMP). NSP-Wisconsin also proposed an Infrastructure Cost Recovery Mechanism (ICRM) rate rider to recover ongoing costs associated with the DIMP. In addition, the filing requested recovery of approximately $129 thousand, or 2.4 percent, through the ICRM, beginning in January 2018. Under the proposal, the ICRM rider would be adjusted annually. No party sought to intervene in the case. In March 2017, NSP-Wisconsin reached a settlement with the MPSC staff that eliminated the ICRM rider in lieu of a multi-year rate plan that includes funding for the DIMP. The settlement authorized a $266 thousand, or 5 percent overall rate increase for 2017, followed by a $140 thousand, or 2.5 percent step increase in January 2018, and another $143 thousand, or 2.5 percent step increase in January 2019. The settlement was based on a 10.0 percent ROE and a 52.56 percent equity ratio. On March 28, 2017 the MPSC issued an order approving the settlement agreement and new rates went into effect on April 1, 2017. Pending Regulatory Proceeding — Federal Energy Regulatory Commission (FERC) Midcontinent Independent System Operator, Inc. (MISO) ROE Complaints/ROE Adder — In November 2013, a group of customers filed a complaint at the FERC against MISO transmission owners (TOs), including NSP-Minnesota and NSP-Wisconsin. The complaint argued for a reduction in the ROE in transmission formula rates in the MISO region from 12.38 percent to 9.15 percent, a prohibition on capital structures in excess of 50 percent equity, and the removal of ROE adders (including those for Regional Transmission Organization (RTO) membership and for being an independent transmission company), effective Nov. 12, 2013. In December 2015, an Administrative Law Judge (ALJ) recommended the FERC approve a ROE of 10.32 percent using a FERC ROE methodology adopted in June 2014, which the FERC upheld in an order issued in September 2016. This ROE is applicable for the 15 month refund period from Nov. 12, 2013 to Feb. 11, 2015, and prospectively from the date of the FERC order. The total prospective ROE is 10.82 percent, which includes a 50 basis point adder for RTO membership. In February 2015, a second complaint seeking to reduce the MISO ROE from 12.38 percent to 8.67 percent prior to any adder was filed with the FERC, resulting in a second period of potential refund from Feb. 12, 2015 to May 11, 2016. The MPUC, the North Dakota Public Service Commission, the South Dakota Public Utilities Commission and the Minnesota Department of Commerce joined a joint complainant/intervenor initial brief recommending an ROE of approximately 8.81 percent. FERC staff recommended a ROE of 8.78 percent. The MISO TOs recommended a ROE of 10.92 percent. In June 2016, the ALJ recommended a ROE of 9.7 percent, the midpoint of the upper half of the discounted cash flow (DCF) range, applying the June 2014 FERC ROE methodology. A decision was expected later in 2017, but could be delayed by the lack of a quorum at the FERC. On April 14, 2017 the D.C. Circuit Court of Appeals vacated and remanded the June 2014 FERC decision, previously made in a New England ROE case. The court decision found that the FERC in that case had not established that the prior ROE was unjust and unreasonable, and that the FERC also failed to adequately support the newly approved ROE. The New England ROE ruling was then the basis for the ROE methodology used in the MISO complaint cases. The court found that the ROE methodology used in the New England ROE case was inadequate because it relied on approaches other than the DCF model. The impact of this court decision on the pending MISO complaint cases is uncertain. As of March 2017, NSP-Minnesota has recognized a current liability for the Nov. 12, 2013 to Feb. 11, 2015 complaint period based on the 10.32 percent ROE provided in the FERC order. This liability is net of refunds processed during the first quarter of 2017. NSP-Minnesota has also recognized a current liability representing the best estimate of the final ROE for the Feb. 12, 2015 to May 11, 2016 complaint period.

Rate Matters

Except to the extent noted below, the circumstances set forth in Note 10 to the consolidated financial statements included in NSP-Wisconsin’s Annual Report on Form 10-K for the year ended Dec. 31, 2016, appropriately represent, in all material respects, the current status of other rate matters, and are incorporated herein by reference.

Recently Concluded Regulatory Proceedings - Michigan Public Service Commission (MPSC)

Michigan 2017 Natural Gas Rate Case — In October 2016, NSP-Wisconsin filed a request with the MPSC to increase base rates for natural gas service by approximately $347 thousand annually, or 6.5 percent. The filing was based on a 2017 forecast test year, a 10.2 percent return on equity (ROE), an equity ratio of 52.56 percent and a forecasted average rate base of approximately $6.4 million. The primary driver of the requested increase is investment in natural gas distribution infrastructure, mainly in conjunction with the NSP-Wisconsin’s Distribution Integrity Management Program (DIMP). NSP-Wisconsin also proposed an Infrastructure Cost Recovery Mechanism (ICRM) rate rider to recover ongoing costs associated with the DIMP. In addition, the filing requested recovery of approximately $129 thousand, or 2.4 percent, through the ICRM, beginning in January 2018. Under the proposal, the ICRM rider would be adjusted annually. No party sought to intervene in the case.

In March 2017, NSP-Wisconsin reached a settlement with the MPSC staff that eliminated the ICRM rider in lieu of a multi-year rate plan that includes funding for the DIMP. The settlement authorized a $266 thousand, or 5 percent overall rate increase for 2017, followed by a $140 thousand, or 2.5 percent step increase in January 2018, and another $143 thousand, or 2.5 percent step increase in January 2019. The settlement was based on a 10.0 percent ROE and a 52.56 percent equity ratio. On March 28, 2017 the MPSC issued an order approving the settlement agreement and new rates went into effect on April 1, 2017.

Pending Regulatory Proceeding — Federal Energy Regulatory Commission (FERC)

Midcontinent Independent System Operator, Inc. (MISO) ROE Complaints/ROE Adder — In November 2013, a group of customers filed a complaint at the FERC against MISO transmission owners (TOs), including NSP-Minnesota and NSP-Wisconsin. The complaint argued for a reduction in the ROE in transmission formula rates in the MISO region from 12.38 percent to 9.15 percent, a prohibition on capital structures in excess of 50 percent equity, and the removal of ROE adders (including those for Regional Transmission Organization (RTO) membership and for being an independent transmission company), effective Nov. 12, 2013.

In December 2015, an Administrative Law Judge (ALJ) recommended the FERC approve a ROE of 10.32 percent using a FERC ROE methodology adopted in June 2014, which the FERC upheld in an order issued in September 2016. This ROE is applicable for the 15 month refund period from Nov. 12, 2013 to Feb. 11, 2015, and prospectively from the date of the FERC order. The total prospective ROE is 10.82 percent, which includes a 50 basis point adder for RTO membership.

In February 2015, a second complaint seeking to reduce the MISO ROE from 12.38 percent to 8.67 percent prior to any adder was filed with the FERC, resulting in a second period of potential refund from Feb. 12, 2015 to May 11, 2016. The MPUC, the North Dakota Public Service Commission, the South Dakota Public Utilities Commission and the Minnesota Department of Commerce joined a joint complainant/intervenor initial brief recommending an ROE of approximately 8.81 percent. FERC staff recommended a ROE of 8.78 percent. The MISO TOs recommended a ROE of 10.92 percent. In June 2016, the ALJ recommended a ROE of 9.7 percent, the midpoint of the upper half of the discounted cash flow (DCF) range, applying the June 2014 FERC ROE methodology. A decision was expected later in 2017, but could be delayed by the lack of a quorum at the FERC.

On April 14, 2017 the D.C. Circuit Court of Appeals vacated and remanded the June 2014 FERC decision, previously made in a New England ROE case. The court decision found that the FERC in that case had not established that the prior ROE was unjust and unreasonable, and that the FERC also failed to adequately support the newly approved ROE. The New England ROE ruling was then the basis for the ROE methodology used in the MISO complaint cases. The court found that the ROE methodology used in the New England ROE case was inadequate because it relied on approaches other than the DCF model. The impact of this court decision on the pending MISO complaint cases is uncertain.

As of March 2017, NSP-Minnesota has recognized a current liability for the Nov. 12, 2013 to Feb. 11, 2015 complaint period based on the 10.32 percent ROE provided in the FERC order. This liability is net of refunds processed during the first quarter of 2017. NSP-Minnesota has also recognized a current liability representing the best estimate of the final ROE for the Feb. 12, 2015 to May 11, 2016 complaint period.