Summary of Significant Accounting Policies Accounting Records (Details) - USD ($) $ in Millions		Dec. 31, 2019	Dec. 31, 2018
Regulatory Assets, Current		$ 48.7	$ 18.5
Regulatory Assets, Noncurrent		306.0	285.8
Regulatory Liability, Noncurrent		1,223.5	1,270.7
Components of Benefit Obligation Regulatory Asset		167.2	188.2
Deferred Storm and Property Reserve Deficiency, Current		2.7
Regulatory Liability, Current		29.0	36.6
Deferred Pension Expenses [Member]
Regulatory Assets, Noncurrent		8.0	6.8
Production Tax Credit Rider [Member]
Regulatory Assets, Current	[1]	1.7	6.9
Dry Scrubber Regulatory Asset [Member]
Regulatory Assets, Noncurrent		20.6	4.5
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		8.1
Pension tracker [Member]
Regulatory Assets, Noncurrent		2.3	0.0
Pension Plans [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset		160.5	185.3
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		11.4
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset		23.3	25.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		2.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset		(16.6)	(22.7)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		(6.1)
Reserve for Tax Refund [Member]
Regulatory Liability, Current	[2]	12.7	15.4
Transmission Cost Recovery Rider [Member]
Regulatory Liability, Current	[2]	0.0	2.7
Income taxes recoverable from customers, net [Member]
Regulatory Liability, Noncurrent		899.2	937.1
SPP Cost Tracker Rider Over Recovery [Member]
Regulatory Liability, Current	[2]	2.6	16.8
Other Regulatory Liabilities [Member]
Regulatory Liability, Noncurrent		5.8	6.8
Regulatory Liability, Current	[2]	6.9	1.4
Oklahoma demand program rider over recovery [Member]
Regulatory Liability, Current	[2]	$ 2.0	$ 0.0

[1] Included in Other Current Assets in the Balance Sheets.

[2] Included in Other Current Liabilities in the Balance Sheets.