Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Return Cash Collateral		$ 0.0	$ 0.0
Reclaim Cash Collateral		5.2	3.0
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		46.0	17.3
Netting	[1]	(27.0)	(10.1)
Derivative Asset, Net		19.0	7.2
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		7.0	1.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		38.0	14.5
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		1.0	0.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		35.6	13.9
Netting	[1]	(27.0)	(10.1)
Derivative Asset, Net		8.6	3.8
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		10.4	3.4
Netting	[1]	0.0	0.0
Derivative Asset, Net		10.4	3.4
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 1 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		7.0	1.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 1 | Natural Gas Commodity
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 | Level 2 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		27.6	11.1
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 2 | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		10.4	3.4
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 3 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		1.0	0.9
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Level 3 | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		13.2	9.6
Netting	[1]	(10.1)	(9.6)
Derivative Asset, Net		3.1	0.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		1.6	0.4
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		9.1	8.1
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		2.5	1.1
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		13.2	9.6
Netting	[1]	(10.1)	(9.6)
Derivative Asset, Net		3.1	0.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Level 1 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		1.6	0.4
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Level 2 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		9.1	8.1
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Level 3 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross		2.5	1.1
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		47.8	21.8
Netting	[1]	(27.0)	(13.1)
Derivative Liability, Net		20.8	8.7
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		7.0	1.7
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		34.5	20.1
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		6.3	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		44.9	18.4
Netting	[1]	(27.0)	(13.1)
Derivative Liability, Net		17.9	5.3
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		2.9	3.4
Netting	[1]	0.0	0.0
Derivative Liability, Net		2.9	3.4
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 1 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		7.0	1.7
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 1 | Natural Gas Commodity
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 | Level 2 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		31.6	16.7
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 2 | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		2.9	3.4
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 3 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		6.3	0.0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Level 3 | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.0	0.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		57.8	62.1
Netting	[1]	(15.3)	(9.6)
Derivative Liability, Net		42.5	52.5
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.9	0.4
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		21.4	47.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		35.5	14.7
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		57.8	62.1
Netting	[1]	(15.3)	(9.6)
Derivative Liability, Net		42.5	52.5
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Level 1 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		0.9	0.4
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Level 2 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		21.4	47.0
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Level 3 | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Gross		$ 35.5	$ 14.7

[1] PSCo nets derivative instruments and related collateral on its consolidated balance sheets when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Sept. 30, 2020 and Dec. 31, 2019. At Sept. 30, 2020 and Dec. 31, 2019, derivative liabilities include no obligations to return cash collateral and the right to reclaim cash collateral of $5.2 million and $3.0 million, respectively. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.