Recurring Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2020		Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Prepayments and other	$ 7		$ 5
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity Contract
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	1		1
Netting	0	[1]	0	[2]
Derivative Asset, Net	1	[3]	1	[4]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity Contract | Level 1
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	0		0
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity Contract | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	1		1
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity Contract | Level 3
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	$ 0		$ 0

[1] NSP-Wisconsin 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 Dec. 31, 2020. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

[2] NSP-Wisconsin 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 Dec. 31, 2019. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

[3] Included in prepayments and other current assets balance of $7 million at Dec. 31, 2020 on the consolidated balance sheets.

[4] Included in prepayments and other current assets balance of $5 million at Dec. 31, 2019 on the consolidated balance sheets.