Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Dec. 31, 2021 USD ($) security	Sep. 30, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss \| security	36	31
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (987)	$ (1,584)
12 months or more, unrealized gross losses	(2,920)	(1,056)
Total unrealized gross losses	(3,907)	(2,640)
Less than 12 months, fair value	141,842	52,397
12 months or more, fair value	140,156	102,324
Total, fair value	281,998	154,721
Held-to-maturity securities
Less than 12 months, unrealized gross losses	0	0
Less than 12 months, fair value	0	0
12 months or more, unrealized gross losses	0	0
12 months or more, fair value	0	0
Total unrealized gross losses	0	0
Total, fair value	0	0
Less than 12 months, unrealized gross losses	(987)	(1,584)
Less than 12 months, fair value	141,842	52,397
12 months or more, unrealized gross losses	(2,920)	(1,056)
12 months or more, fair value	140,156	102,324
Total unrealized gross losses	(3,907)	(2,640)
Total fair value	281,998	154,721
U.S. government and agency securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(88)
12 months or more, unrealized gross losses	0
Total unrealized gross losses	(88)
Less than 12 months, fair value	13,970
12 months or more, fair value	0
Total, fair value	13,970
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(144)	(15)
12 months or more, unrealized gross losses	(855)	(614)
Total unrealized gross losses	(999)	(629)
Less than 12 months, fair value	71,099	4,639
12 months or more, fair value	87,409	70,058
Total, fair value	158,508	74,697
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(755)	(1,569)
12 months or more, unrealized gross losses	(2,065)	(442)
Total unrealized gross losses	(2,820)	(2,011)
Less than 12 months, fair value	56,773	47,758
12 months or more, fair value	52,747	32,266
Total, fair value	$ 109,520	$ 80,024

No definition available.

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456