Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Regulatory Assets and Liabilities Disclosure [Abstract]

Regulatory Assets

Components of regulatory assets:

(Millions of Dollars)	See Note(s)	Remaining Amortization Period	Dec. 31, 2018				Dec. 31, 2017
Regulatory Assets			Current		Noncurrent		Current		Noncurrent
Pension and retiree medical obligations	9	Various	$	12.6	$	222.1	$	12.7	$	223.0
Excess deferred taxes - TCJA	7	Various	—		55.9		—		44.7
Recoverable deferred taxes on AFUDC recorded in plant		Plant lives	—		27.9		—		23.9
Net AROs (a)	1, 10	Plant lives	—		25.7		—		24.2
Losses on reacquired debt		Term of related debt	0.8		21.9		0.8		22.7
Conservation programs (b)	1	One to two years	0.7		0.6		2.7		0.7
Other		Various	11.9		12.1		15.3		23.7
Total regulatory assets			$	26.0	$	366.2	$	31.5	$	362.9

^(a)  Includes amounts recorded for future recovery of AROs.

^(b)  Includes costs for conservation programs, as well as incentives allowed in certain jurisdictions.

Regulatory Liabilities

Components of regulatory liabilities:

(Millions of Dollars)	See Note(s)	Remaining Amortization Period	Dec. 31, 2018				Dec. 31, 2017
Regulatory Liabilities			Current		Noncurrent		Current		Noncurrent
Deferred income tax adjustments and TCJA refunds (a)	7	Various	$	2.2	$	569.8	$	—	$	568.6
Plant removal costs	1, 10	Plant lives	—		187.7		—		196.9
Revenue subject to refund		One to two years	11.3		8.1		6.8		6.5
Gain from asset sales		Various	—		2.4		—		2.5
Deferred electric energy costs		Less than one year	56.5		—		48.5		—
Contract valuation adjustments (b)	1, 8	Less than one year	14.7		—		12.7		—
Other		Various	1.1		12.9		0.8		10.1
Total regulatory liabilities			$	85.8	$	780.9	$	68.8	$	784.6

^(a)  Includes the revaluation of recoverable/regulated plant ADIT and revaluation impact of non-plant ADIT due to the TCJA.

^(b)  Includes the fair value of certain long-term PPAs used to meet energy capacity requirements.