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Short-Term and Long-Term Borrowings - Narrative (Details)	1 Months Ended		9 Months Ended
	May 31, 2022 USD ($)	Nov. 30, 2021 USD ($)	Sep. 30, 2021 USD ($) issuance	Jun. 10, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Instrument [Line Items]
Line Limit			$ 340,000,000		$ 340,000,000
Long-term debt			767,000,000		767,169,000
Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 230,000,000
Long-term debt			$ 0
Number of issuances | issuance			2
Senior Notes | Forecast
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 90,000,000	$ 140,000,000
OTC Credit Agreement
Debt Instrument [Line Items]
Line Limit			$ 170,000,000		170,000,000
Debt, weighted average interest rate (as a percent)			1.59%
OTC Credit Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Line Limit			$ 170,000,000
OTC Credit Agreement | Letter of Credit
Debt Instrument [Line Items]
Line Limit			40,000,000
Line of credit facility, increased current borrowing capacity, accordion provision			290,000,000
OTP Credit Agreement
Debt Instrument [Line Items]
Line Limit			$ 170,000,000		$ 170,000,000
Debt, weighted average interest rate (as a percent)			1.33%
OTP Credit Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Line Limit			$ 170,000,000
OTP Credit Agreement | Letter of Credit
Debt Instrument [Line Items]
Line Limit			50,000,000
Line of credit facility, increased current borrowing capacity, accordion provision			$ 250,000,000
Senior Unsecured Notes 2.74%, Series A, Due November 29, 2031 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 40,000,000
Interest rate (as a percent)				2.74%
Senior Unsecured Notes 3.69%, Series B, Due November 29, 2051 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 100,000,000
Interest rate (as a percent)				3.69%
Senior Unsecured Notes 3.77%, Series A, Due May 20, 2052 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount				$ 90,000,000
Interest rate (as a percent)				3.77%
Minimum
Debt Instrument [Line Items]
Interest and dividend coverage ratio			1.50
Minimum | OTC Credit Agreement | Prime Rate, Federal Funds Rate or LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percent)			2.00%
Minimum | OTC Credit Agreement | Benchmark Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percent)			0.00%
Maximum
Debt Instrument [Line Items]
Debt to total capitalization ratio			60.00%
Maximum | OTC Credit Agreement | Prime Rate, Federal Funds Rate or LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percent)			0.125%
Maximum | OTC Credit Agreement | Benchmark Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate (as a percent)			1.75%

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