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Fair Value Measurements - Securities in Unrealized Loss Positions (Details) $ in Thousands	Dec. 31, 2020 USD ($) security	Sep. 30, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Number of debt securities with unrealized loss | security	33	51
Available-for-sale securities
Less than 12 months, unrealized gross losses	$ (1,745)	$ (5,833)
12 months or more, unrealized gross losses	(2,108)	(3,781)
Total unrealized gross losses	(3,853)	(9,614)
Less than 12 months, fair value	317,909	675,518
12 months or more, fair value	195,084	285,161
Total, fair value	512,993	960,679
Held-to-maturity securities
Less than 12 months, unrealized gross losses	(4)	(52)
12 months or more, unrealized gross losses	0	0
Total unrealized gross losses	(4)	(52)
Less than 12 months, fair value	5,553	6,853
12 months or more, fair value	0	0
Total, fair value	5,553	6,853
Less than 12 months, unrealized gross losses	(1,749)	(5,885)
12 months or more, unrealized gross losses	(2,108)	(3,781)
Total unrealized gross losses	(3,857)	(9,666)
Less than 12 months, fair value	323,462	682,371
12 months or more, fair value	195,084	285,161
Total fair value	518,546	967,532
Corporate debt securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(173)	(74)
12 months or more, unrealized gross losses	0	(405)
Total unrealized gross losses	(173)	(479)
Less than 12 months, fair value	45,811	45,875
12 months or more, fair value	0	24,596
Total, fair value	45,811	70,471
Asset-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(398)	(5,481)
12 months or more, unrealized gross losses	(1,699)	(3,027)
Total unrealized gross losses	(2,097)	(8,508)
Less than 12 months, fair value	219,539	587,746
12 months or more, fair value	141,478	204,369
Total, fair value	361,017	792,115
Mortgage-backed securities
Available-for-sale securities
Less than 12 months, unrealized gross losses	(1,174)	(278)
12 months or more, unrealized gross losses	(409)	(349)
Total unrealized gross losses	(1,583)	(627)
Less than 12 months, fair value	52,559	41,897
12 months or more, fair value	53,606	56,196
Total, fair value	$ 106,165	$ 98,093

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