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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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