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Income Taxes (Narrative) (Details) $ in Millions | 3 Months Ended | 9 Months Ended | 60 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 USD ($) generator | Jun. 30, 2019 USD ($) | Sep. 30, 2018 | Sep. 30, 2019 generator | Sep. 30, 2018 | Dec. 31, 2018 USD ($)
Income Tax Credits [Line Items] |  |  |  |  |  |
Effective tax rate (as a percent) | 16.90% |  | 16.40% | 22.20% | 13.50% |
Reversal of state income taxes payable | $ 6.4 |  |  |  |  |
DC Solar Tax Credit Investments |  |  |  |  |  |
Income Tax Credits [Line Items] |  |  |  |  |  |
Reversal of certain previously claimed tax credits |  | $ 30.1 |  |  |  |
Investment tax credits |  |  |  |  |  | $ 53.9
Deferred tax liability |  |  |  |  |  | $ 5.7
Tax basis reduction (as a percent) |  |  |  |  |  | 50.00%
Tax credit claimed, net of tax basis reduction |  |  |  |  |  | $ 48.2
Amount reversed of previously claimed tax credit |  | 33.6 |  |  |  |
Amount reversed of deferred tax liability |  | $ 3.5 |  |  |  |
Number of mobile service generators identified | generator | 0 |  |  | 0 |  |

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