Regulated Operations (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2016	Dec. 31, 2015
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause over recoveries		$ 1.4	$ 61.3
Fuel clause under recoveries		0.5	0.0
Current Regulatory Assets		59.4	46.5
Non-Current Regulatory Assets		404.8	402.2
Current Regulatory Liabilities		5.2	68.8
Non-Current Regulatory Liabilities		292.0	273.6
Prior Period Reclassification Adjustment		1.0
Other Regulatory Liability [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	3.8	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		260.1	254.9
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		30.9	17.7
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	48.9	36.6
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	7.6	5.4
Non-Current Regulatory Assets		16.8	17.3
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		235.0	242.2
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		60.0	56.7
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		43.4	43.6
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		35.9	27.6
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		13.7	14.8
SPP Cost Tracker Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[3]	$ 2.4	$ 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.