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SJW Group and Connecticut Water Service Merger Agreement (Details) - $ / shares | Mar. 14, 2018 | Sep. 30, 2018 | Dec. 31, 2017
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Business Combinations [Abstract] |  |  |
Conversion rate of CTWS shares to SJW Group (in shares) | 70.00 |  |
SJW Group common stock, par value (usd per share) |  | $ 0.001 | $ 0.001

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