Revenue recognition (Details 7) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Revenue recognition [Abstract]
Accrued unbilled revenue at Montana-Dakota, Cascade and Intermountain	$ 85.9	$ 80.2

The amount due for services rendered or products shipped, but not yet billed, recognized in conformity with revenue recognition criteria. This element is distinct from unbilled contracts receivables because this is based on noncontract transactions.

No definition available.

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