Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Sep. 30, 2023
Amortized Cost, Available-for-sale securities
Amortized cost	$ 2,508,223	$ 2,118,616
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	6,273	2,910
Gross unrealized loss	(85,727)	(126,429)
Fair Value, Available-for-sale securities
Fair value	$ 2,428,769	$ 1,995,097
Yield, Available-for-sale securities
Yield	4.81%	4.64%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 447,638	$ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	170	0
Held-to-maturity securities, gross unrealized losses	(49,803)	(68,398)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 398,005	$ 355,188
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.17%	2.88%
Investments	$ 2,955,861	$ 2,542,202
Gross unrealized gains on investments	6,443	2,910
Gross unrealized losses on investments	(135,530)	(194,827)
Total financial assets	$ 2,826,774	$ 2,350,285
Yield on investments	4.51%	4.35%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 6,530	$ 3,501
Amortized cost, 1 to 5 years	5,528	18,894
Amortized cost, 5 to 10 years	151,376	87,922
Amortized cost, over 10 years	128,092	106,340
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	3	0
Gross unrealized gains, 1 to 5 years	3	0
Gross unrealized gains, 5 to 10 years	1,205	177
Gross unrealized gains, over 10 years	571	831
Gross unrealized losses, Within 1 year	(4)	(36)
Gross unrealized losses, 1 to 5 years	(233)	(563)
Gross unrealized losses, 5 to 10 years	0	0
Gross unrealized losses, over 10 years	(296)	(13)
Fair Value, Available-for-sale securities
Fair value, within 1 year	6,529	3,465
Fair value, 1 to 5 years	5,298	18,331
Fair value, 5 to 10 years	152,581	88,099
Fair value, over 10 years	$ 128,367	$ 107,158
Yield, Available-for-sale securities
Yield, within 1 year	5.84%	6.06%
Yield, 1 to 5 years	2.82%	4.70%
Yield, 5 to 10 years	6.01%	5.76%
Yield, over 10 years	6.33%	5.84%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 12,369	$ 18,579
Amortized cost, 5 to 10 years	17,411	36,875
Amortized cost, over 10 years	479,681	539,911
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	63	2
Gross unrealized gains, over 10 years	672	578
Gross unrealized losses, 1 to 5 years	(334)	(715)
Gross unrealized losses, 5 to 10 years	0	(99)
Gross unrealized losses, over 10 years	(2,050)	(7,115)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	12,035	17,864
Fair value, 5 to 10 years	17,474	36,778
Fair value, over 10 years	$ 478,303	$ 533,374
Yield, Available-for-sale securities
Yield, 1 to 5 years	6.10%	6.06%
Yield, 5 to 10 years	6.23%	6.11%
Yield, over 10 years	6.33%	6.35%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 25,043
Amortized cost, 1 to 5 years	119,108	$ 151,893
Amortized cost, 5 to 10 years	112,327	113,221
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	0
Gross unrealized gains, 1 to 5 years	882	895
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized losses, Within 1 year	(436)
Gross unrealized losses, 1 to 5 years	(163)	(1,787)
Gross unrealized losses, 5 to 10 years	(15,594)	(21,700)
Fair Value, Available-for-sale securities
Fair value, within 1 year	24,607
Fair value, 1 to 5 years	119,827	151,001
Fair value, 5 to 10 years	$ 96,733	$ 91,521
Yield, Available-for-sale securities
Yield, within 1 year	3.50%
Yield, 1 to 5 years	5.58%	5.14%
Yield, 5 to 10 years	3.87%	3.87%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 5,697	$ 5,720
Amortized cost, over 10 years	29,803	29,832
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, over 10 years	274	361
Gross unrealized losses, 5 to 10 years	(514)	(701)
Gross unrealized losses, over 10 years	(243)	(550)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	5,183	5,019
Fair value, over 10 years	$ 29,834	$ 29,643
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	$ 1,415,258	$ 1,005,928
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	2,600	66
Gross unrealized losses, without single maturity date	(65,860)	(93,150)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 1,351,998	$ 912,844
Yield, Available-for-sale securities
Yield, without single maturity date	4.01%	3.39%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 447,638	$ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	170	0
Held-to-maturity securities, gross unrealized losses	(49,803)	(68,398)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 398,005	$ 355,188
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.17%	2.88%