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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Sep. 30, 2022
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,178,717	$ 2,162,737
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	1,210	1,308
Gross unrealized loss	(120,090)	(113,008)
Fair Value, Available-for-sale securities
Fair value	$ 2,059,837	$ 2,051,037
Yield, Available-for-sale securities
Yield	3.99%	3.36%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 453,443	$ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	23	22
Held-to-maturity securities, gross unrealized losses	(51,411)	(56,461)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 402,055	$ 406,860
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%
Investments	$ 2,632,160	$ 2,626,036
Gross unrealized gains on investments	1,233	1,330
Gross unrealized losses on investments	(171,501)	(169,469)
Total financial assets	$ 2,461,892	$ 2,457,897
Yield on investments	3.80%	3.28%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 36,531	$ 40,403
Amortized cost, over 10 years	64,060
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, over 10 years	223
Gross unrealized losses, 1 to 5 years	(925)	(1,049)
Gross unrealized losses, over 10 years	0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	35,606	$ 39,354
Fair value, over 10 years	$ 64,283
Yield, Available-for-sale securities
Yield, 1 to 5 years	4.33%	3.03%
Yield, over 10 years	3.90%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 21,033	$ 22,527
Amortized cost, 5 to 10 years	67,783	82,962
Amortized cost, over 10 years	627,579	668,482
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	0	4
Gross unrealized gains, over 10 years	365	783
Gross unrealized losses, 1 to 5 years	(1,524)	(1,141)
Gross unrealized losses, 5 to 10 years	(1,025)	(547)
Gross unrealized losses, over 10 years	(17,639)	(6,069)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	19,509	21,386
Fair value, 5 to 10 years	66,758	82,419
Fair value, over 10 years	$ 610,305	$ 663,196
Yield, Available-for-sale securities
Yield, 1 to 5 years	4.58%	3.27%
Yield, 5 to 10 years	5.01%	3.53%
Yield, over 10 years	5.12%	3.86%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 75,000	$ 75,000
Amortized cost, 1 to 5 years	151,531	151,411
Amortized cost, 5 to 10 years	114,116	114,414
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	29	4
Gross unrealized gains, 1 to 5 years	232	0
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized losses, Within 1 year	(212)	(200)
Gross unrealized losses, 1 to 5 years	(3,435)	(2,748)
Gross unrealized losses, 5 to 10 years	(20,774)	(24,124)
Fair Value, Available-for-sale securities
Fair value, within 1 year	74,817	74,804
Fair value, 1 to 5 years	148,328	148,663
Fair value, 5 to 10 years	$ 93,342	$ 90,290
Yield, Available-for-sale securities
Yield, within 1 year	5.08%	3.74%
Yield, 1 to 5 years	4.49%	3.59%
Yield, 5 to 10 years	3.87%	3.87%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 5,743	$ 5,751
Amortized cost, over 10 years	29,861	29,871
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, over 10 years	350	400
Gross unrealized losses, 5 to 10 years	(360)	(361)
Gross unrealized losses, over 10 years	(540)	(699)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	5,383	5,390
Fair value, over 10 years	$ 29,671	$ 29,572
Yield, Available-for-sale securities
Yield, 5 to 10 years	3.00%	3.00%
Yield, over 10 years	5.85%	5.85%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 985,480	$ 971,916
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	11	117
Gross unrealized losses, without single maturity date	(73,656)	(76,070)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 911,835	$ 895,963
Yield, Available-for-sale securities
Yield, without single maturity date	2.98%	2.81%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 453,443	$ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	23	22
Held-to-maturity securities, gross unrealized losses	(51,411)	(56,461)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 402,055	$ 406,860
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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