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Share-Based Compensation, TSR Liability Awards (Details) shares in Thousands, $ in Thousands | 12 Months Ended
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Dec. 31, 2018 USD ($) Parties shares | Dec. 31, 2017 USD ($) shares | Dec. 31, 2016 USD ($) shares
Equity Instruments Other than Options Activity [Roll Forward] |  |  |
Granted (in shares) | 500 | 503 | 522
Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |
Award Vesting Period (in years) | 3 years |  |
Number of Utilities in Peer Group | Parties | 22 |  |
TSR Liability Awards |  |  |
Equity Instruments Other than Options Activity [Roll Forward] |  |  |
Granted (in shares) | 239 | 240 | 264
Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |
Award Vesting Period (in years) | 3 years |  |
Amount of cash used to settle TSR liability awards | $ | $ 8,000 |  |
Awards settled (in shares) | 482 | 454 | 354
Settlement amount (cash and common stock) | $ | $ 21,534 | $ 19,083 | $ 13,724
TSR Liability Awards | Minimum |  |  |
Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |
Percentage payout for TSR liability awards | 0.00% |  |
TSR Liability Awards | Maximum |  |  |
Equity Instruments Other than Options, Additional Disclosures [Abstract] |  |  |
Percentage payout for TSR liability awards | 200.00% |  |

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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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