Document and Entity Information Document - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	BLACK HILLS POWER INC
Entity Central Index Key	0000012400
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Document Fiscal Year End Focus	2018
Document Fiscal Period Focus	FY
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Common Stock Share Outstanding		23,416,396
Entity Well-known seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Public Float			$ 0