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Regulatory Assets and Liabilities - Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities
Current liabilities	$ 41.8	$ 33.4
Regulatory liabilities	3,718.6	1,563.8
Total regulatory liabilities	3,760.4	1,597.2
2017 Tax legislation impact and income tax related
Regulatory Liabilities
Total regulatory liabilities	2,134.1	0.0
Removal costs
Regulatory Liabilities
Total regulatory liabilities	1,294.9	1,262.7
Unrecognized pension and OPEB costs
Regulatory Liabilities
Total regulatory liabilities	114.2	63.0
Mines deferral
Regulatory Liabilities
Total regulatory liabilities	95.1	70.2
Energy costs refundable through rate adjustments
Regulatory Liabilities
Total regulatory liabilities	42.0	88.7
Uncollectible expense
Regulatory Liabilities
Total regulatory liabilities	24.7	36.1
Derivatives
Regulatory Liabilities
Total regulatory liabilities	11.0	41.1
Other, net
Regulatory Liabilities
Total regulatory liabilities	44.4	$ 35.4
Utility operations
Regulatory Liabilities
Increase (decrease) in deferred income taxes	$ 2,450.0

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- Definition

The increase (decrease) during the reporting period in the account that represents the temporary difference that results from Income or Loss that is recognized for accounting purposes but not for tax purposes and vice versa.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
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- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
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The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 980
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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