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Revenue recognition (Details 7) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Accounting Policies [Abstract] |  |
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain | $ 99.7 us-gaap_UnbilledReceivablesCurrent | $ 107.4 us-gaap_UnbilledReceivablesCurrent

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- Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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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
Name: | us-gaap_UnbilledReceivablesCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant