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Short-Term and Long-Term Borrowings - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Jun. 10, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Line Limit	$ 340,000,000
Debt, weighted average interest rate (as a percent)	1.42%		1.61%
Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 230,000,000
Proceeds from issuance of debt	$ 140,000,000
Minimum
Debt Instrument [Line Items]
Interest and dividend coverage ratio	1.50
Maximum
Debt Instrument [Line Items]
Debt to total capitalization ratio	60.00%
Benchmark rate | Minimum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	0.00%
Benchmark rate | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
OTC Credit Agreement
Debt Instrument [Line Items]
Line Limit	$ 170,000,000
Line of credit facility, increased current borrowing capacity, accordion provision	290,000,000
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
OTP Credit Agreement
Debt Instrument [Line Items]
Line Limit	170,000,000
Line of credit facility, increased current borrowing capacity, accordion provision	250,000,000
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
Series 2021A Senior Unsecured Notes
Debt Instrument [Line Items]
Interest rate (as a percent)	2.74%
Series 2021A Senior Unsecured Notes | Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount		$ 40,000,000
Interest rate (as a percent)		2.74%
Series 2021B Senior Unsecured Notes
Debt Instrument [Line Items]
Interest rate (as a percent)	3.69%
Series 2021B Senior Unsecured Notes | 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%

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