Stock-Based Compensation (Tables)	9 Months Ended
Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of PSU Transactions	Number Weighted of Average Share Units Fair ValueNonvested share units at beginning of period 283,007 $ 34.57Granted 152,068 43.18Performance criteria adjustment 27,643 53.62Forfeited (5,588) 45.84Share units issued (141,328) 31.36Nonvested share units at end of period 315,802 41.51
Summary Of RSU Transactions	Number Weighted of Average Stock Units Fair ValueNonvested stock units at beginning of period 163,906 $ 40.80Granted 85,829 44.44Stock units vested and issued (49,692) 35.33Forfeited (3,325) 43.30Nonvested stock units at end of period 196,718 43.72
Summary Of Stock Option Transactions	Weighted Weighted Average Average Aggregate Exercise Remaining Intrinsic Shares Price Life (years) ValueOutstanding at beginning of period 948,270 $ 35.22 Granted - - Forfeited (7,219) 35.91 Expired (984) 33.93 Exercised (97,659) 33.72 Outstanding at end of period 842,408 $ 35.22 7.1 9,009,081 Exercisable at end of period 609,410 $ 35.17 7.0 6,650,661
Performance Share Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Compensation Costs	Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020Stock-based compensation within operations and maintenance expenses $ 1,285 $ 918 $ 4,216 $ 2,364Income tax benefit 365 252 1,191 660
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Compensation Costs	Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020Stock-based compensation within operations and maintenance expenses $ 724 $ 550 $ 2,089 $ 1,582Income tax benefit 205 148 586 438
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Compensation Costs	Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020Stock-based compensation within operations and maintenance expenses $ 94 $ 279 $ 395 $ 1,046Income tax benefit 27 79 113 296
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Compensation Costs	Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020Stock-based compensation within operations and maintenance expenses $ 11 $ 103 $ 117 $ 220Income tax benefit 3 30 34 64
Summary Of Restricted Stock Transactions	Number Weighted of Average Shares Fair ValueNonvested restricted stock at beginning of period 13,228 $ 34.02Granted 1,068 46.83Vested (13,228) 34.02Nonvested restricted stock at end of period 1,068 $ 46.83
Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary Of Compensation Costs	Three Months Ended Nine Months Ended September 30, September 30, 2021 2020 2021 2020Stock-based compensation within operations and maintenance expenses $ 175 $ 175 $ 525 $ 520Income tax benefit 51 50 152 150
Summary Of Nonvested Share Activity	Number Weighted of Average Stock Awards Fair ValueNonvested stock awards at beginning of period - $ -Granted 11,487 45.71Vested (11,487) (45.71)Nonvested stock awards at end of period - -

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