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Pension Plan and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost–Benefit Earned During the Period $ 6,621 $ 5,491 $ 6,459
Interest Cost on Projected Benefit Obligation 13,053 14,412 13,452
Expected Return on Assets (22,021) (21,297) (21,199)
Amortization of Prior Service Cost:      
From Regulatory Asset 0 5 16
From Other Comprehensive Income1 0 9 0
Amortization of Net Actuarial Loss:      
From Regulatory Asset 8,924 4,642 7,135
From Other Comprehensive Income1 220 114 183
Net Periodic Pension Cost 6,797 3,376 6,046
Pension Plan | Service costs included in OTP capital expenditures      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 1,842 1,365 1,542
Pension Plan | Service costs included in electric operation and maintenance expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 4,621 3,994 4,756
Pension Plan | Service costs included in other nonelectric expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 159 132 161
Pension Plan | Nonservice costs capitalized      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 48 (526) (99)
Pension Plan | Nonservice costs included in nonservice cost components of postretirement benefits      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 127 (1,589) (314)
Executive Survivor and Supplemental Retirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost–Benefit Earned During the Period 179 418 408
Interest Cost on Projected Benefit Obligation 1,449 1,735 1,589
Amortization of Prior Service Cost:      
From Regulatory Asset 0 5 20
From Other Comprehensive Income1 0 17 34
Amortization of Net Actuarial Loss:      
From Regulatory Asset 93 124 206
From Other Comprehensive Income1 341 348 722
Net Periodic Pension Cost 2,062 2,647 2,979
Executive Survivor and Supplemental Retirement Plan | Service costs included in electric operation and maintenance expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 0 104 99
Executive Survivor and Supplemental Retirement Plan | Service costs included in other nonelectric expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 179 314 309
Executive Survivor and Supplemental Retirement Plan | Nonservice costs included in nonservice cost components of postretirement benefits      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 1,883 2,229 2,571
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost–Benefit Earned During the Period 1,847 1,286 1,526
Interest Cost on Projected Benefit Obligation 2,393 3,083 2,583
Amortization of Prior Service Cost:      
From Regulatory Asset (4,677) 0 0
From Other Comprehensive Income1 (115) 0 0
Amortization of Net Actuarial Loss:      
From Regulatory Asset 4,206 1,571 1,648
From Other Comprehensive Income1 104 38 42
Net Periodic Pension Cost 3,758 5,978 5,799
Effect of Medicare Part D Subsidy 1,123 (179) (470)
Other Postretirement Benefits Plan | Service costs included in OTP capital expenditures      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 514 320 364
Other Postretirement Benefits Plan | Service costs included in electric operation and maintenance expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 1,289 935 1,124
Other Postretirement Benefits Plan | Service costs included in other nonelectric expenses      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 44 31 38
Other Postretirement Benefits Plan | Nonservice costs capitalized      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost 532 1,167 1,020
Other Postretirement Benefits Plan | Nonservice costs included in nonservice cost components of postretirement benefits      
Amortization of Net Actuarial Loss:      
Net Periodic Pension Cost $ 1,379 $ 3,525 $ 3,253