Commitments and Contingencies - Dividend Restrictions (Details) (Utilities Group [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014
Utilities Group [Member]
Related Party Transaction [Line Items]
Restricted Net Assets for Subsidiaries	$ 73

Amount of Restricted Net Assets (Equity) for Subsidiaries as of the end of the reporting period.

No definition available.

Name:	bkh_RestrictedNetAssetsForSubsidiaries
Namespace Prefix:	bkh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant