Fair Value of Financial Assets and Liabilities - Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Derivatives, Fair Value
Return Cash Collateral		$ 0.0		$ 0.0		$ 0.0
Reclaim Cash Collateral		0.0		0.0		0.0
Commodity Contract [Member]
Changes in Level 3 Commodity Derivatives
Balance at beginning of period		16.3	$ 3.1	11.7	$ 14.7
Purchases		9.0	17.1	20.8	21.0
Settlements		(11.9)	(13.1)	(16.8)	(19.7)
Net gains recognized as regulatory assets and liabilities		2.3	15.1	0.0	6.2
Balance at end of period		15.7	22.2	15.7	22.2
Transfers Level 3, Net		0.0	$ 0.0	0.0	$ 0.0
Other Current Assets
Derivatives, Fair Value
Derivative asset, net		18.8		18.8		15.0
Other Noncurrent Assets
Derivatives, Fair Value
Derivative asset, net		11.1		11.1		12.6
Other Current Liabilities
Derivatives, Fair Value
Derivative liability, net		3.6		3.6		3.7
Other Noncurrent Liabilities
Derivatives, Fair Value
Derivative liability, net		11.0		11.0		12.8
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value
Derivative asset, gross		15.9		15.9		11.8
Asset, netting	[1]	(0.2)		(0.2)		0.0
Derivative asset, net		15.7		15.7		11.8
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 1
Derivatives, Fair Value
Derivative asset, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 2
Derivatives, Fair Value
Derivative asset, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 3
Derivatives, Fair Value
Derivative asset, gross		15.9		15.9		11.8
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		15.9		15.9		11.8
Asset, netting	[1]	(0.2)		(0.2)		0.0
Derivative asset, net		15.7		15.7		11.8
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 1 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 2 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 3 | Electric Commodity
Derivatives, Fair Value
Derivative asset, gross		15.9		15.9		11.8
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value
Derivative liability, gross		0.2		0.2		0.1
Liabilities, netting	[1]	(0.2)		(0.2)		0.0
Derivative liability, net		0.0		0.0		0.1
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 1
Derivatives, Fair Value
Derivative liability, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 2
Derivatives, Fair Value
Derivative liability, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 3
Derivatives, Fair Value
Derivative liability, gross		0.2		0.2		0.1
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity
Derivatives, Fair Value
Derivative liability, gross		0.2		0.2		0.1
Liabilities, netting	[1]	(0.2)		(0.2)		0.0
Derivative liability, net		0.0		0.0		0.1
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 1 | Electric Commodity
Derivatives, Fair Value
Derivative liability, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 2 | Electric Commodity
Derivatives, Fair Value
Derivative liability, gross		0.0		0.0		0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 3 | Electric Commodity
Derivatives, Fair Value
Derivative liability, gross		0.2		0.2		0.1
Fair Value, Measurements, Nonrecurring | Other Current Assets | PPAs
Derivatives, Fair Value
Derivative asset, net	[2]	3.1		3.1		3.2
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | PPAs
Derivatives, Fair Value
Derivative asset, net	[2]	11.1		11.1		12.6
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | PPAs
Derivatives, Fair Value
Derivative liability, net	[2]	3.6		3.6		3.6
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | PPAs
Derivatives, Fair Value
Derivative liability, net	[2]	$ 11.0		$ 11.0		$ 12.8

[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 June 30, 2020 and Dec. 31, 2019. At both June 30, 2020 and Dec. 31, 2019, 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.