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Note 6 - Investments in Marketable Securities (Details Textual) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Marketable Securities, Accumulated Unrealized Gain (Loss)	$ 83,012,000		$ 83,012,000		$ 140,692,000
Marketable Securities, Unrealized Gain (Loss), Total	$ 77,211,000	$ 8,497,000	$ 57,680,000	$ 20,143,000

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

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

X

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(7)(c))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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