Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Return Cash Collateral		$ 0.0	$ 0.0
Commodity Contract [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period		14.7	12.7	$ 2.0
Purchases		26.7	32.3	41.2
Settlements		(34.2)	(41.6)	(55.8)
Gains (losses) recognized as regulatory assets		4.5	11.3	25.3
Balance at end of period		11.7	14.7	12.7
Transfers Between Levels, Net		0.0	0.0	$ 0.0
Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		15.0	17.8
Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		12.6	15.8
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Net		3.7	3.6
Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Net		12.8	16.4
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Netting	[1]	0.0	(0.2)
Derivative Asset, Net		11.8	14.7
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Netting	[1]	0.0	(0.2)
Derivative Asset, Net		11.8	14.7
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity Contract | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		11.8	14.9
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Netting	[1]	0.0	(0.2)
Derivative Liability, Net		0.1	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Netting	[1]	0.0	(0.2)
Derivative Liability, Net		0.1	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity Contract | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.1	0.2
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Net	[2]	3.2	3.1
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Asset, Net		12.6	15.8
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Net	[2]	3.6	3.6
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | Purchased Power Agreements
Derivatives, Fair Value [Line Items]
Derivative Liability, Net	[2]	$ 12.8	$ 16.4

[1] SPS nets derivative instruments and related collateral in its balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2019 and 2018. At both Dec. 31, 2019 and 2018, derivative assets and liabilities include no obligations to return cash collateral or rights to reclaim cash collateral. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

[2] During 2006, SPS qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.