Recurring Fair Value Measures (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Collateral, Obligation to Return Cash, Offset		$ 0		$ 0		$ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset		3,200,000		3,200,000		6,500,000
Commodity Trading
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings		500,000		1,500,000
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers into Level 3		0	$ 0	0	$ 0
Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability		11,700,000		11,700,000		34,600,000
Fair Value Measured on a Recurring Basis | Other Current Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		20,000,000.0		20,000,000.0		70,800,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	(10,000,000.0)		(10,000,000.0)		(28,200,000)
Derivative Asset		10,000,000.0		10,000,000.0		42,600,000
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,400,000		1,400,000		2,300,000
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		18,400,000		18,400,000		68,400,000
Fair Value Measured on a Recurring Basis | Other Current Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		200,000		200,000		100,000
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		6,300,000		6,300,000		1,600,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	(5,600,000)		(5,600,000)		(400,000)
Derivative Asset		700,000		700,000		1,200,000
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		500,000		500,000		0
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		5,700,000		5,700,000		1,600,000
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		100,000		100,000		0
Fair Value Measured on a Recurring Basis | Other Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		25,000,000.0		25,000,000.0		66,600,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1]	(13,300,000)		(13,300,000)		(34,700,000)
Derivative Liability		11,700,000		11,700,000		31,900,000
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		1,400,000		1,400,000		2,400,000
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		23,500,000		23,500,000		64,200,000
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		100,000		100,000		0
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		60,400,000		60,400,000		1,100,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1]	(5,600,000)		(5,600,000)		(500,000)
Derivative Liability		54,800,000		54,800,000		600,000
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		400,000		400,000		0
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		60,000,000.0		60,000,000.0		1,100,000
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		15,200,000		15,200,000		67,400,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	(10,000,000.0)		(10,000,000.0)		(28,200,000)
Derivative Asset		5,200,000		5,200,000		39,200,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Commodity Trading | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,400,000		1,400,000		2,300,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Commodity Trading | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		13,600,000		13,600,000		65,000,000.0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Commodity Trading | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		200,000		200,000		100,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		4,800,000		4,800,000		3,400,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	0		0		0
Derivative Asset		4,800,000		4,800,000		3,400,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Natural Gas Commodity | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Natural Gas Commodity | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		4,800,000		4,800,000		3,400,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Assets | Natural Gas Commodity | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Assets | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		6,300,000		6,300,000		1,600,000
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	[1]	(5,600,000)		(5,600,000)		(400,000)
Derivative Asset		700,000		700,000		1,200,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Assets | Commodity Trading | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		500,000		500,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Assets | Commodity Trading | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		5,700,000		5,700,000		1,600,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Assets | Commodity Trading | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		100,000		100,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		20,600,000		20,600,000		66,600,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1]	(13,300,000)		(13,300,000)		(34,700,000)
Derivative Liability		7,300,000		7,300,000		31,900,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Commodity Trading | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		1,400,000		1,400,000		2,400,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Commodity Trading | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		19,100,000		19,100,000		64,200,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Commodity Trading | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		100,000		100,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		4,400,000		4,400,000		0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1]	0		0		0
Derivative Liability		4,400,000		4,400,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Natural Gas Commodity | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Natural Gas Commodity | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		4,400,000		4,400,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Current Liabilities | Natural Gas Commodity | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Liabilities | Commodity Trading
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		60,400,000		60,400,000		1,100,000
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	[1]	(5,600,000)		(5,600,000)		(500,000)
Derivative Liability		54,800,000		54,800,000		600,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Liabilities | Commodity Trading | Level 1
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		400,000		400,000		0
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Liabilities | Commodity Trading | Level 2
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		60,000,000.0		60,000,000.0		1,100,000
Fair Value Measured on a Recurring Basis | Other Derivative Instruments | Other Noncurrent Liabilities | Commodity Trading | Level 3
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		0		0		0
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | PPAs
Derivatives, Fair Value [Line Items]
Derivative Liability	[2]	$ 0		$ 0		$ 2,700,000

[1] PSCo nets derivative instruments and related collateral in its consolidated 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 Sept. 30, 2019 and Dec. 31, 2018. At both Sept. 30, 2019 and Dec. 31, 2018, derivative assets and liabilities include no obligations to return cash collateral. At Sept. 30, 2019 and Dec. 31, 2018, derivative assets and liabilities include the rights to reclaim cash collateral of $3.2 million and $6.5 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.

[2] During 2006, PSCo 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.