Regulated Operations (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2016	Dec. 31, 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause over recoveries		$ 41.3	$ 61.3
Current Regulatory Assets		63.2	46.5
Non-Current Regulatory Assets		401.9	402.2
Current Regulatory Liabilities		45.8	68.8
Non-Current Regulatory Liabilities		283.9	273.6
Prior Period Reclassification Adjustment		1.0
Other Regulatory Liability [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	4.5	7.5
Non-Current Regulatory Liabilities	[2]	1.0	1.0
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		256.2	254.9
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		26.7	17.7
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	43.9	36.6
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	11.8	5.4
Non-Current Regulatory Assets		16.9	17.3
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		237.3	242.2
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		58.3	56.7
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		43.5	43.6
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		31.8	27.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		14.1	14.8
SPP Cost Tracker Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	$ 7.5	$ 4.5

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

[2] Prior year amount of $1.0 million reclassified from deferred other liabilities to Non-Current Regulatory Liabilities.

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