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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,921,988	$ 2,090,070
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	10,988	50,829
Gross unrealized loss	(23,371)	(2,640)
Fair Value, Available-for-sale securities
Fair value	$ 1,909,605	$ 2,138,259
Yield, Available-for-sale securities
Yield	1.95%	1.69%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 301,221	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	170	13,522
Held-to-maturity securities, gross unrealized losses	(7,037)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 294,354	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.16%	3.17%
Investments	$ 2,223,209	$ 2,456,095
Gross unrealized gains on investments	11,158	64,351
Gross unrealized losses on investments	(30,408)	(2,640)
Total financial assets	$ 2,203,959	$ 2,517,806
Yield on investments	2.11%	1.89%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 50,441	$ 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	27	240
Gross unrealized gains, 5 to 10 years		136
Gross unrealized losses, 1 to 5 years	(659)	0
Gross unrealized losses, 5 to 10 years		0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	$ 49,809	47,579
Fair value, 5 to 10 years		$ 14,200
Yield, Available-for-sale securities
Yield, 1 to 5 years	1.07%	0.44%
Yield, 5 to 10 years		2.05%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 18,908	$ 19,730
Amortized cost, 5 to 10 years	62,883	71,207
Amortized cost, over 10 years	807,611	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	121	412
Gross unrealized gains, over 10 years	2,144	14,069
Gross unrealized losses, 1 to 5 years	(602)	(434)
Gross unrealized losses, 5 to 10 years	(202)	(97)
Gross unrealized losses, over 10 years	(1,437)	(98)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	18,306	19,296
Fair value, 5 to 10 years	62,802	71,522
Fair value, over 10 years	$ 808,318	$ 987,863
Yield, Available-for-sale securities
Yield, 1 to 5 years	1.08%	0.52%
Yield, 5 to 10 years	0.79%	0.57%
Yield, over 10 years	1.28%	0.94%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year		$ 225,928
Amortized cost, 1 to 5 years	$ 226,170	86,802
Amortized cost, 5 to 10 years	115,010	28,804
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year		7,860
Gross unrealized gains, 1 to 5 years	2,301	1,345
Gross unrealized gains, 5 to 10 years	0	249
Gross unrealized losses, Within 1 year		0
Gross unrealized losses, 1 to 5 years	0	0
Gross unrealized losses, 5 to 10 years	(9,482)	0
Fair Value, Available-for-sale securities
Fair value, within 1 year		233,788
Fair value, 1 to 5 years	228,471	88,147
Fair value, 5 to 10 years	$ 105,528	$ 29,053
Yield, Available-for-sale securities
Yield, within 1 year		1.57%
Yield, 1 to 5 years	1.79%	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	$ 1,509	$ 1,493
Amortized cost, 5 to 10 years	5,766	5,781
Amortized cost, over 10 years	29,890	29,909
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	3	17
Gross unrealized gains, 5 to 10 years	0	294
Gross unrealized gains, over 10 years	1,456	2,490
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 5 to 10 years	(156)	0
Gross unrealized losses, over 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	1,512	1,510
Fair value, 5 to 10 years	5,610	6,075
Fair value, over 10 years	$ 31,346	$ 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	$ 603,800	$ 585,121
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	4,936	23,717
Gross unrealized losses, without single maturity date	(10,833)	(2,011)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 597,903	$ 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	$ 301,221	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	170	13,522
Held-to-maturity securities, gross unrealized losses	(7,037)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 294,354	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.16%	3.17%

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