Regulatory Matters - Regulatory Assets and Liabilities (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets and Liabilities Currently Earning a Return		No regulatory assets or liabilities are currently earning a return.
Current Regulatory Asset		$ 10.6	$ 16.3
Non-Current Regulatory Asset		372.0	357.3
Total Regulatory Assets		382.6	373.6
Non-Current Regulatory Liability		105.0	94.2
Wholesale and Retail Contra AFUDC [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liability	[1]	58.0	42.9
Plant Removal Obligations [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liability		22.1	22.8
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liability	[2]	6.1	13.4
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liability	[3]	0.9	3.5
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liability		17.9	11.6
Deferred Fuel Adjustment Clause [Member]
Regulatory Assets and Liabilities [Line Items]
Current Regulatory Asset	[4]	10.6	16.3
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset	[3]	219.3	223.9
Income Taxes [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset	[2]	64.2	46.6
Cost Recovery Riders [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset	[5]	58.0	59.7
Asset Retirement Obligation [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset	[6]	21.6	17.8
PPACA Income Tax Deferral [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset		5.0	5.0
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Asset		$ 3.9	$ 4.3

[1] Wholesale and Retail Contra AFUDC represents amortization to offset AFUDC Equity and Debt recorded during the construction period of our cost recovery rider projects prior to placing the projects in service. The regulatory liability will decrease over the remaining depreciable life of the related asset.

[2] These costs represent the difference between deferred income taxes recognized for financial reporting purposes and amounts previously billed to our customers. This balance will decrease over the remaining life of the related temporary differences and flow through current income taxes.

[3] Defined benefit pension and other postretirement items included in our Regulated Operations, which are otherwise required to be recognized in accumulated other comprehensive income as actuarial gains and losses as well as prior service costs and credits, are recognized as regulatory assets or regulatory liabilities on the Consolidated Balance Sheet. The asset or liability will decrease as the deferred items are amortized and recognized as components of net periodic benefit cost. (See Note 17. Pension and Other Postretirement Benefit Plans.)

[4] Current regulatory assets are included in Prepayments and Other on the Consolidated Balance Sheet.

[5] The cost recovery rider regulatory assets are revenues not yet collected from our customers primarily due to capital expenditures related to the Bison Wind Energy Center, investment in CapX2020 projects, and the Boswell Unit 4 environmental upgrade and are recognized in accordance with the accounting standards for alternative revenue programs.

[6] Asset retirement obligations will accrete and be amortized over the lives of the related property with asset retirement obligations.