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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Sep. 30, 2021
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,216,651	$ 2,090,070
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	2,612	50,829
Gross unrealized loss	(68,531)	(2,640)
Fair Value, Available-for-sale securities
Fair value	$ 2,150,732	$ 2,138,259
Yield, Available-for-sale securities
Yield	2.49%	1.69%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 477,884	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	115	13,522
Held-to-maturity securities, gross unrealized losses	(30,170)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 447,829	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.89%	3.17%
Investments	$ 2,694,535	$ 2,456,095
Gross unrealized gains on investments	2,727	64,351
Gross unrealized losses on investments	(98,701)	(2,640)
Total financial assets	$ 2,598,561	$ 2,517,806
Yield on investments	2.63%	1.89%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 46,584	$ 47,339
Amortized cost, 5 to 10 years		14,064
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	240
Gross unrealized gains, 5 to 10 years		136
Gross unrealized losses, 1 to 5 years	(1,122)	0
Gross unrealized losses, 5 to 10 years		0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	$ 45,462	47,579
Fair value, 5 to 10 years		$ 14,200
Yield, Available-for-sale securities
Yield, 1 to 5 years	1.87%	0.44%
Yield, 5 to 10 years		2.05%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 23,396	$ 19,730
Amortized cost, 5 to 10 years	94,022	71,207
Amortized cost, over 10 years	731,942	973,892
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	61	412
Gross unrealized gains, over 10 years	780	14,069
Gross unrealized losses, 1 to 5 years	(1,066)	(434)
Gross unrealized losses, 5 to 10 years	(376)	(97)
Gross unrealized losses, over 10 years	(10,726)	(98)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	22,330	19,296
Fair value, 5 to 10 years	93,707	71,522
Fair value, over 10 years	$ 721,996	$ 987,863
Yield, Available-for-sale securities
Yield, 1 to 5 years	2.00%	0.52%
Yield, 5 to 10 years	2.01%	0.57%
Yield, over 10 years	2.35%	0.94%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 75,000	$ 225,928
Amortized cost, 1 to 5 years	151,290	86,802
Amortized cost, 5 to 10 years	114,712	28,804
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	83	7,860
Gross unrealized gains, 1 to 5 years	77	1,345
Gross unrealized gains, 5 to 10 years	0	249
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 1 to 5 years	(1,667)	0
Gross unrealized losses, 5 to 10 years	(17,571)	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	75,083	233,788
Fair value, 1 to 5 years	149,700	88,147
Fair value, 5 to 10 years	$ 97,141	$ 29,053
Yield, Available-for-sale securities
Yield, within 1 year	2.44%	1.57%
Yield, 1 to 5 years	2.68%	4.64%
Yield, 5 to 10 years	3.87%	1.95%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 1,515	$ 1,493
Amortized cost, 5 to 10 years	5,758	5,781
Amortized cost, over 10 years	29,880	29,909
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	0	17
Gross unrealized gains, 5 to 10 years	0	294
Gross unrealized gains, over 10 years	756	2,490
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 5 to 10 years	(246)	0
Gross unrealized losses, over 10 years	(432)	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	1,515	1,510
Fair value, 5 to 10 years	5,512	6,075
Fair value, over 10 years	$ 30,204	$ 32,399
Yield, Available-for-sale securities
Yield, within 1 year	0.00%	0.00%
Yield, 5 to 10 years	0.16%	0.22%
Yield, over 10 years	5.85%	5.85%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 942,552	$ 585,121
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	855	23,717
Gross unrealized losses, without single maturity date	(35,325)	(2,011)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 908,082	$ 606,827
Yield, Available-for-sale securities
Yield, without single maturity date	2.60%	2.60%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 477,884	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	115	13,522
Held-to-maturity securities, gross unrealized losses	(30,170)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 447,829	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.89%	3.17%

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