Borrowings and Other Financing Instruments, Long-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount (Premium), Net	$ (4.0)	$ (2.0)
Unamortized Debt Issuance Expense	(20.0)	(18.0)
Long-term Debt, Current Maturities	0.0	0.0
Long-term Debt, Excluding Current Maturities	2,126.1	1,829.9
Deferred Finance Costs, Noncurrent, Net	20.0	18.0
First Mortgage Bonds, Series due:
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,800.0	$ 1,500.0
Debt Instrument, Maturity Date Range, End	Jan. 01, 2048
Debt Instrument, Maturity Date Range, Start	Jun. 15, 2024
First Mortgage Bonds, Series due: \| Series Due June 15, 2048 [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.40%
Face Amount	$ 300.0
Debt Instrument, Maturity Date	Nov. 15, 2048
First Mortgage Bonds, Series due: \| Series Due August 15, 2047
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)		3.70%
Face Amount		$ 450.0
Debt Instrument, Maturity Date		Aug. 15, 2047
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 350.0	$ 350.0
Debt Instrument, Maturity Date Range, End	Oct. 01, 2036
Debt Instrument, Maturity Date Range, Start	Oct. 01, 2033
Combined Total
Debt Instrument [Line Items]
Long-term Debt, Excluding Current Maturities	$ 2,126.0	$ 1,830.0
Maximum \| First Mortgage Bonds, Series due:
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	4.50%	4.50%
Maximum \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	6.00%	8.75%
Minimum \| First Mortgage Bonds, Series due:
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	3.30%	3.30%
Minimum \| Unsecured Debt [Member]
Debt Instrument [Line Items]
Interest Rate, Stated Percentage (in hundredths)	6.00%	6.00%
Credit Facility
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jun. 30, 2021

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount (premium).

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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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