Summary of Significant Accounting Policies, Regulated Operations (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Schedule of Regulatory Assets and Liabilities [Line Items]
Prior Period Reclassification Adjustment		$ 1.1
Fuel clause under recoveries		1.6	$ 68.3
Fuel clause over recoveries		34.5	0.0
Total Current Regulatory Assets		41.7	98.2
Total Non-Current Regulatory Assets		389.7	410.4
Total Current Regulatory Liabilities		49.2	24.4
Total Non-Current Regulatory Liabilities		277.0	263.0
Crossroads Wind Farm Rider Over Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	7.2	10.3
Smart Grid rider over recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	5.3	12.5
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Liabilities	[1]	2.2	1.6
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities		257.1	248.1
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Liabilities		19.9	14.9
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	[2]	30.3	19.7
Other [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Current Regulatory Assets	[2],[3]	9.8	10.2
Total Non-Current Regulatory Assets	[3]	17.0	15.7
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		241.6	261.1
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		55.7	56.1
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		43.7	43.9
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		16.6	17.5
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Total Non-Current Regulatory Assets		$ 15.1	$ 16.1

[1] Included in Other Current Liabilities on the Condensed Consolidated Balance Sheets.

[2] Included in Other Current Assets on the Condensed Consolidated Balance Sheets.

[3] Prior year amount of $1.1 million reclassified from Non-Current Other assets to Current Other assets.