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Marketable Securities - Additional Information (Detail) - USD ($)	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Marketable Securities [Line Items]
Changes in net unrealized gains and (losses)	$ 1,190,000	$ (2,425,000)
Transfers of investments between Level 1 and Level 2	0	0
Payments to Acquire Marketable Securities	$ 35,313,000	68,444,000
Government Securities [Member]
Marketable Securities [Line Items]
Payments to Acquire Marketable Securities		$ 2,000,000

X

- Definition

Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bb)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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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.

+ Details

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X

- Definition

The sum of the realized and unrealized net gain (loss) on trading securities during the period.

+ References

No definition available.

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X

- Definition

Amount of cash outflow for purchase of marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26853-111562

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