Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2017

Regulatory Assets and Liabilities Disclosure [Abstract]

Schedule of Regulatory Assets [Table Text Block]

The regulatory assets and regulatory liabilities resulting from our application of the provisions of ASC Topic 980, Regulated Operations, included in the accompanying Consolidated Balance Sheet at December 31, 2017 and December 31, 2016 are as follows (in millions):

Regulatory Assets	2017		2016
Grossed-up deferred taxes on equity funds used during construction	$	37.9	$	73.2
Asset retirement obligations	168.7		117.2
Asset retirement costs - Eminence	49.5		53.9
Deferred taxes - asset	3.8		4.8
Deferred cash out	42.5		48.2
Deferred gas costs	6.0		—
Fuel cost	61.4		51.3
Other	3.7		2.5
Total Regulatory Assets	$	373.5	$	351.1

Schedule of Regulatory Liabilities [Table Text Block]

Regulatory Liabilities	2017		2016
Negative salvage	$	409.7	$	357.1
Deferred taxes - liability	471.1		—
Sentinel meter station depreciation	6.3		6.2
Postretirement benefits other than pension	73.9		63.0
Electric power cost	13.3		6.3
Pension - deferred collections	32.5		21.3
Other	0.3		4.6
Total Regulatory Liabilities	$	1,007.1	$	458.5