Fair Value of Financial Assets and Liabilities - Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Prepayments and other	$ 5	$ 7
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	0	1
Netting	0	0	[1]
Derivative Asset, Net	0	1	[2]
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity | 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 | Level 2
Derivatives, Fair Value [Line Items]
Derivative Asset, Gross	0	1
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity | 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 March 31, 2021 and Dec. 31, 2020. 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] Included in prepayments and other current assets balance of $5 million at March 31, 2021 and $7 million at Dec. 31, 2020 on the consolidated balance sheets.