Summary of Significant Accounting Policies Accounting Records (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets, Current		$ 18.5	$ 40.8
Regulatory Assets, Noncurrent		285.8	283.0
Regulatory Liability, Noncurrent		1,270.7	1,283.4
Components of Benefit Obligation Regulatory Asset		188.2	177.2
Deferred Storm and Property Reserve Deficiency, Current		2.7
Regulatory Liability, Current		36.6	3.9
Deferred Pension Expenses [Member]
Regulatory Assets, Noncurrent		6.8	5.1
Production Tax Credit Rider [Member]
Regulatory Assets, Current	[1]	6.9	0.0
Dry Scrubber Regulatory Asset [Member]
Regulatory Assets, Noncurrent		4.5	0.0
Regulatory Asset [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		10.4
Pension Plans [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset		185.3	172.4
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		13.8
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Components of Benefit Obligation Regulatory Asset		25.6	33.6
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		2.7
Other Postretirement Benefit Plans, Defined Benefit [Member] | Prior Service Cost [Member]
Components of Benefit Obligation Regulatory Asset		(22.7)	(28.8)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year		(6.1)
SPP Cost Tracker Rider Under Recovery [Member]
Regulatory Liability, Current	[2]	16.8	0.0
Reserve for Tax Refund [Member]
Regulatory Liability, Current	[2]	15.4	0.0
Transmission Cost Recovery Rider [Member]
Regulatory Liability, Current	[2]	2.7	0.2
Income taxes recoverable from customers, net [Member]
Regulatory Liability, Noncurrent		$ 937.1	$ 955.5

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

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