CONSOLIDATED STATEMENTS OF CAPITALIZATION - USD ($) shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Schedule of Capitalization, Long-term Debt [Line Items]
Common Stock, Value	$ 2.0	$ 2.0
Premium on common stock	1,122.6	1,129.3
Retained earnings	2,544.6	3,036.1
Accumulated other comprehensive loss, net of tax	$ (32.1)	$ (27.9)
Treasury Stock, Common, Shares	0.1	0.0
Treasury Stock, Value	$ (5.3)	$ 0.0
Total stockholders' equity	3,631.8	4,139.5
Unamortized debt expense	(25.3)	(24.2)
Total long-term debt	3,494.4	3,195.2
Less: long-term debt due within one year	0.0	0.0
Total long-term debt (excluding long-term debt due within one year)	3,494.4	3,195.2
Total capitalization (including long-term debt due within one year)	$ 7,126.2	$ 7,334.7
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Common Stock, Shares Authorized	450.0	450.0
Common Stock, Shares, Outstanding	200.1	200.1
Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Common Stock, Value	$ 100.9	$ 100.9
Premium on common stock	938.6	935.7
Retained earnings	2,936.1	2,921.7
Total stockholders' equity	3,975.6	3,958.3
Unamortized debt expense	(25.3)	(24.2)
Unamortized discount	(10.1)	(10.5)
Total long-term debt	3,494.4	3,195.2
Less: long-term debt due within one year	0.0	0.0
Total long-term debt (excluding long-term debt due within one year)	3,494.4	3,195.2
Total capitalization (including long-term debt due within one year)	$ 7,470.0	$ 7,153.5
Common Stock, Par or Stated Value Per Share	$ 2.50	$ 2.50
Common Stock, Shares Authorized	100.0	100.0
Common Stock, Shares, Outstanding	40.4	40.4
Garfield Industrial Authority Bond [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Jan. 01, 2025
Muskogee Industrial Authority Bond Due 2025 [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Jan. 01, 2025
Muskogee Industrial Authority Bond Due 2027 [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Jun. 01, 2027
Senior Notes [Member] \| Series Due July 15, 2027 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 125.0	$ 125.0
Debt Instrument, Interest Rate, Stated Percentage	6.65%
Debt Instrument, Maturity Date	Jul. 15, 2027
Senior Notes [Member] \| Series Due April 15, 2028 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 100.0	100.0
Debt Instrument, Interest Rate, Stated Percentage	6.50%
Debt Instrument, Maturity Date	Apr. 15, 2028
Senior Notes [Member] \| Series Due August 15, 2028 [Member] [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 400.0	400.0
Debt Instrument, Interest Rate, Stated Percentage	3.80%
Senior Notes [Member] \| Series Due March 15, 2030 \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 300.0	300.0
Debt Instrument, Interest Rate, Stated Percentage	3.30%
Debt Instrument, Maturity Date	Mar. 15, 2030
Senior Notes [Member] \| Series Due January 15, 2036 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 110.0	110.0
Debt Instrument, Interest Rate, Stated Percentage	5.75%
Debt Instrument, Maturity Date	Jan. 15, 2036
Senior Notes [Member] \| Series Due February 1, 2038 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 200.0	200.0
Debt Instrument, Interest Rate, Stated Percentage	6.45%
Debt Instrument, Maturity Date	Feb. 01, 2038
Senior Notes [Member] \| Series Due June 1, 2040 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 250.0	250.0
Debt Instrument, Interest Rate, Stated Percentage	5.85%
Debt Instrument, Maturity Date	Jun. 01, 2040
Senior Notes [Member] \| Series Due May 15, 2041 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 250.0	250.0
Debt Instrument, Interest Rate, Stated Percentage	5.25%
Debt Instrument, Maturity Date	May 15, 2041
Senior Notes [Member] \| Series Due May 1, 2043 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 250.0	250.0
Debt Instrument, Interest Rate, Stated Percentage	3.90%
Debt Instrument, Maturity Date	May 01, 2043
Senior Notes [Member] \| Series Due March 15, 2044 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 250.0	250.0
Debt Instrument, Interest Rate, Stated Percentage	4.55%
Debt Instrument, Maturity Date	Mar. 15, 2044
Senior Notes [Member] \| Series Due December 15, 2044 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 250.0	250.0
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Debt Instrument, Maturity Date	Dec. 15, 2044
Senior Notes [Member] \| Series due August 15, 2047 [Member] [Domain] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 300.0	300.0
Debt Instrument, Interest Rate, Stated Percentage	3.85%
Debt Instrument, Maturity Date	Aug. 15, 2047
Senior Notes [Member] \| Series due April 1, 2047 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 300.0	300.0
Debt Instrument, Interest Rate, Stated Percentage	4.15%
Debt Instrument, Maturity Date	Apr. 01, 2047
Senior Notes [Member] \| Series Due April 1, 2030 \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 300.0	0.0
Debt Instrument, Interest Rate, Stated Percentage	3.25%
Debt Instrument, Maturity Date	Apr. 01, 2030
Senior Notes [Member] \| Series due August 15, 2028 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Maturity Date	Aug. 15, 2028
Long-term Debt [Member] \| Due August 31, 2062 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 9.4	9.5
Debt Instrument, Interest Rate, Stated Percentage	3.80%
Debt Instrument, Maturity Date	Aug. 31, 2062
Debentures Subject to Mandatory Redemption [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Total long-term debt	$ 135.4
Debentures Subject to Mandatory Redemption [Member] \| Garfield Industrial Authority Bond [Member] \| Minimum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.28%
Debentures Subject to Mandatory Redemption [Member] \| Garfield Industrial Authority Bond [Member] \| Maximum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.35%
Debentures Subject to Mandatory Redemption [Member] \| Garfield Industrial Authority Bond [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 47.0	47.0
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2025 [Member] \| Minimum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.33%
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2025 [Member] \| Maximum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.31%
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2025 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 32.4	32.4
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2027 [Member] \| Minimum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	0.28%
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2027 [Member] \| Maximum [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.35%
Debentures Subject to Mandatory Redemption [Member] \| Muskogee Industrial Authority Bond Due 2027 [Member] \| Og and E [Member]
Schedule of Capitalization, Long-term Debt [Line Items]
Long-term Debt, Gross	$ 56.0	$ 56.0

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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