Financing Activities (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2019	Jun. 30, 2019
Secured Debt Montana Due June 2049 [Member]
Debt Instrument [Line Items]
Face amount		$ 150
Interest rate, stated percentage		3.98%
Maturity date		Sep. 30, 2049
Secured Debt Montana Due June 2049 [Member]
Debt Instrument [Line Items]
Issuance date		Jun. 30, 2019
Face amount		$ 50
Forecast [Member] \| Secured Debt Montana Due September 2049 [Member]
Debt Instrument [Line Items]
Issuance date	Sep. 30, 2019
Face amount	$ 100

Face (par) amount of debt instrument at time of issuance.

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

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- 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.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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