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Acquisitions - Natural Gas Storage Facility in Michigan (Details) - Natural gas storage facility in Michigan - Subsequent event - USD ($) $ in Millions | Jan. 01, 2017 | Jan. 31, 2017
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Business Acquisition [Line Items] |  |
Total purchase price | $ 225 |
Expected acquisition costs |  | $ 5
Percentage of storage needs provided |  | 33.00%

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