Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2019

Fair Value Disclosures [Abstract]

Gross Notional Amounts of Commodity Forwards and Options

Gross notional amounts of commodity options at Dec. 31:

(Amounts in Millions) (a) (b)	2019	2018
MMBtu of natural gas	0.1	1.2

^(a)  Amounts are not reflective of net positions in the underlying commodities.

^(b) 

Notional amounts for options are included on a gross basis, but are weighted for the probability of exercise.

Financial Impact of Qualifying Cash Flow Hedges on Accumulated Other Comprehensive Loss

(Millions of Dollars)	2019		2018			2017
Accumulated other comprehensive loss related to cash flow hedges at Jan. 1	$	—	$	(0.1	)	$	(0.1	)
After-tax net realized losses on derivative transactions reclassified into earnings	—		0.1			—
Accumulated other comprehensive loss related to cash flow hedges at Dec. 31	$	—	$	—		$	(0.1	)

Derivative Assets and Liabilities Measured at Fair Value on a Recurring Basis by Hierarchy Level

	Dec. 31, 2019
	Fair Value						Fair Value Total		Netting (a)
(Millions of Dollars)	Level 1		Level 2		Level 3						Total (b)
Current derivative assets
Natural gas commodity	$	—	$	0.5	$	—	$	0.5	$	—	$	0.5

^(a)  NSP-Wisconsin 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 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.

^(b)  Included in the prepayments balance of $4.3 million at Dec. 31, 2019 in the consolidated balance sheet.

	Dec. 31, 2018
	Fair Value						Fair Value Total		Netting (a)
(Millions of Dollars)	Level 1		Level 2		Level 3						Total (b)
Current derivative assets
Natural gas commodity	$	—	$	0.2	$	—	$	0.2	$	—	$	0.2

^(a)  NSP-Wisconsin 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 Dec. 31, 2018. The counterparty netting excludes settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.

^(b) 

Included in the prepayments balance of $3.3 million at Dec. 31, 2018 in the consolidated balance sheet.

Carrying Amount and Fair Value of Long-term Debt

As of Dec. 31, other financial instruments for which the carrying amount did not equal fair value:

	2019				2018
(Millions of Dollars)	Carrying Amount		Fair Value		Carrying Amount		Fair Value
Long-term debt, including current portion	$	808.0	$	924.3	$	807.5	$	850.4