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Revenue recognition (Details 6) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Accounting Policies [Abstract] |  |
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain | $ 112.7 | $ 117.7

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- Definition Amount received for services rendered and products shipped, but not yet billed, for non-contractual agreements due within one year or the normal operating cycle, if longer. + References No definition available. + Details
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