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v3.21.2

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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