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Long-Term Debt: Dividend Restrictions (Details) (Utilities Group [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Utilities Group [Member]
 
Debt Instrument [Line Items]  
Restricted Net Assets for Subsidiaries $ 315us-gaap_AmountOfRestrictedNetAssetsForConsolidatedAndUnconsolidatedSubsidiaries
/ bkh_BusinessGroupAxis
= bkh_UtilitiesGroupMember