Commitments and Contingencies Supply Agreements (Details) (Basin Electric [Member])	9 Months Ended
Sep. 30, 2013 MW
Basin Electric [Member]
Supply Commitment [Line Items]
Number of Megawatts Sold Under Long-Term Contract	40
Long-term Contract To Sell Electric Power, Date of Contract Expiration	Sep. 30, 2014

Date of contract expiration of arrangements in which the entity must commit resources to supply goods or services to one or more customers.

No definition available.

Name:	bkh_LongTermContractToSellElectricPowerDateOfContractExpiration
Namespace Prefix:	bkh_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration