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Borrowings and Other Financing Instruments Borrowings and Other Financing Instruments, Dividend and Other Capital-Related Restrictions (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
PSCo |  |
Debt Instrument [Line Items] |  |
Maximum additional long term debt authorized for issuance | $ 1,100.0 |
Maximum additional short term debt authorized for issuance | $ 800.0 |
NSP-Minnesota |  |
Debt Instrument [Line Items] |  |
Equity to total capitalization ratio, low end of range (in hundredths) | 47.10% |
Equity to total capitalization ratio, high end of range (in hundredths) | 57.50% |
Equity to total capitalization ratio | 52.30% |
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions | $ 1,000.0 |
Capitalization, Short term debt, long term debt and equity | 10,700.0 |
Maximum total capitalization | 11,500.0 |
Maximum additional short term debt authorized for issuance | $ 1,725.0 |
Maximum Percentage of long term debt to total capitalization | 5293000.00% |
Maximum percentage of short term debt to total capitalization (in hundredths) | 15.00% |
NSP-Wisconsin |  |
Debt Instrument [Line Items] |  |
Minimum calendar year average equity to total capitalization ratio authorized by state commission | 51.50% |
Equity to total capitalization ratio | 51.80% |
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions | $ 11.5 | [1]
Capitalization, Short term debt, long term debt and equity | 1,700.0 |
Maximum annual dividends that can be paid if equity capitalization ratio condition is not met | 55.0 |
Maximum additional long term debt authorized for issuance | 0.0 | [2]
Maximum additional short term debt authorized for issuance | $ 150.0 |
SPS |  |
Debt Instrument [Line Items] |  |
Equity to total capitalization ratio (excluding short-term debt), low end of range (in hundredths) | 45.00% | [3]
Equity to total capitalization ratio (excluding short-term debt), high end of range (in hundredths) | 55.00% | [3]
Equity to total capitalization ratio (excluding short-term debt) (in hundredths) | 54.40% | [3]
Unrestricted Retained Earnings Per State Regulatory Commissions Dividend Restrictions | $ 605.7 | [4]
Capitalization, Short term debt, long term debt and equity | 4,700.0 |
Maximum additional long term debt authorized for issuance | 0.0 | [2]
Maximum additional short term debt authorized for issuance | $ 600.0 |
[1] | NSP-Wisconsin cannot pay annual dividends in excess of approximately $55 million if its average equity-to-total capitalization ratio falls below the commission authorized level.
[2] | SPS and NSP-Wisconsin will file for additional long-term debt authorization.
[3] | SPS excludes short-term debt.
[4] | SPS may not pay a dividend that would cause it to lose its investment grade bond rating.

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