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Investments - Narrative (Details) - Fixed income securities - USD ($) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
ARIZONA PUBLIC SERVICE COMPANY |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Coal reclamation escrow account | $ 30,000,000 |
Proceeds from sale of securities | 4,000,000 | $ 0
4CA |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Coal reclamation escrow account | $ 2,000,000 |

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