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Note 8 - Fair Value of Financial Instruments (Details Textual) - USD ($)	1 Months Ended	9 Months Ended
	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024	Sep. 30, 2024
Marketable Securities, Accumulated Unrealized Gain (Loss)		$ 303,917,000		$ 219,597,000
Marketable Security, Unrealized Gain (Loss)		84,320,000	$ 62,472,000
Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale	$ 40,579,000	$ 0	$ 40,579,000
Marketable Security, Realized Gain (Loss)	$ 14,261,000

X

- Definition

Amount of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in debt and equity securities.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of realized gain (loss) on investment in marketable security.

+ References

No definition available.

+ Details

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

X

- Definition

Amount of unrealized gain (loss) on investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2

+ Details

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Namespace Prefix:	us-gaap_
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Period Type:	duration

X

- Definition

Amount of cash inflow from sale, maturity, prepayment and call of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481830/320-10-45-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-11

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

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