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
Environmental remediation costs	1, 10	Various	$	16.0	$	135.2	$	16.0	$	136.1
Pension and retiree medical obligations	9	Various	5.5		85.1		5.7		87.5
Excess deferred taxes - TCJA	7	Various	—		25.2		—		22.6
Recoverable deferred taxes on AFUDC recorded in plant		Plant lives	—		16.9		—		14.3
State commission adjustments		Plant lives	0.8		16.8		0.7		15.9
Losses on reacquired debt		Term of related debt	0.2		2.4		0.7		2.7
Other		Various	0.1		3.9		—		3.1
Total regulatory assets			$	22.6	$	285.5	$	23.1	$	282.2

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	$	0.2	$	238.3	$	—	$	236.1
Plant removal costs	1, 10	Plant lives	—		157.7		—		146.4
Deferred electric production and natural gas costs		Less than one year	13.4		—		13.9		—
United States Department of Energy settlement		Less than one year	6.2		—		5.3		—
Other		Various	1.1		4.1		1.5		4.3
Total regulatory liabilities			$	20.9	$	400.1	$	20.7	$	386.8

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