Commercial Paper, Maximum Borrowing Capacity

No definition available.

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Namespace Prefix:	bkh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number Of One-Year Extension Options

+ References

No definition available.

+ Details

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The net cash inflow or outflow for borrowing having initial term of repayment within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-04(a))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShortTermDebtLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
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- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
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Data Type:	na
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- Details

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Notes Payable, Current Maturities and Debt: Schedule of Short-term Debt and Narrative (Details)		3 Months Ended
	Jul. 30, 2018 USD ($) credit_extension	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Short-term Debt [Line Items]
Notes payable		$ 164,650,000	$ 164,200,000	$ 185,620,000
Letters of Credit		14,006,000	15,830,000	22,310,000
Commercial paper, maximum borrowing capacity		750,000,000
Net (payments) borrowings of short-term debt		(20,970,000)	(47,100,000)
Commercial Paper
Short-term Debt [Line Items]
Notes payable		$ 164,650,000	164,200,000	185,620,000
Debt instrument, term		397 days
Net (payments) borrowings of short-term debt		$ (21,000,000)
Short-term interest rate		2.70%
Revolving Credit Facility
Short-term Debt [Line Items]
Notes payable		$ 0	0	0
Letters of Credit		$ 14,006,000	$ 15,830,000	$ 22,310,000
Current borrowing capacity	$ 750,000,000
Number of one-year extension options \| credit_extension	2
Debt instrument, term	1 year
Maximum borrowing capacity	$ 1,000,000,000.0
Commitment fee		0.175%
Revolving Credit Facility \| Base Rate
Short-term Debt [Line Items]
Interest rate		0.125%
Revolving Credit Facility \| Eurodollar
Short-term Debt [Line Items]
Interest rate		1.125%
Revolving Credit Facility \| Letter of Credit
Short-term Debt [Line Items]
Interest rate		1.125%