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Note 6 - Investments in Marketable Securities - Summary of Investments (Details) - Common Stock [Member] - USD ($)	Mar. 31, 2020	Sep. 30, 2019
Aggregate fair value	$ 136,901,000	$ 194,581,000
Amortized/adjusted cost basis	53,889,000	53,889,000
Pretax net unrealized gains	$ 83,012,000	$ 140,692,000

X

- Definition

Amount of unrealized gain (loss) in accumulated other comprehensive income (AOCI) on investments in 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 investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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Balance Type:	debit
Period Type:	instant

X

- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

+ Details

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

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type: