Regulated Operations (Details) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Fuel clause under recoveries		$ 73.4	$ 51.3
Current Regulatory Assets		142.2	121.8
Non-Current Regulatory Assets		403.1	404.8
Current Regulatory Liabilities		13.4	12.3
Non-Current Regulatory Liabilities		306.7	299.7
Other Regulatory Liability [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Liabilities	[1]	13.4	12.3
Non-Current Regulatory Liabilities		1.5	1.4
Accrued removal obligations, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		268.6	262.8
Pension tracker
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Liabilities		36.6	35.5
Oklahoma demand program rider under recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	45.6	51.0
Other Regulatory Asset [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	8.6	9.5
Non-Current Regulatory Assets		16.8	17.6
Benefit obligations regulatory asset
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		228.9	232.6
Income taxes recoverable from customers, net
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		65.6	62.3
Smart Grid
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		38.2	43.2
Deferred storm expenses
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		40.5	35.7
Unamortized loss on reacquired debt
Schedule of Regulatory Assets and Liabilities [Line Items]
Non-Current Regulatory Assets		13.1	13.4
SPP Cost Tracker Rider Under Recovery [Member]
Schedule of Regulatory Assets and Liabilities [Line Items]
Current Regulatory Assets	[2]	$ 14.6	$ 10.0

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

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