Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Sep. 30, 2023
Amortized Cost, Available-for-sale securities
Amortized cost	$ 2,101,314	$ 2,118,616
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	2,075	2,910
Gross unrealized loss	(84,944)	(126,429)
Fair Value, Available-for-sale securities
Fair value	$ 2,018,445	$ 1,995,097
Yield, Available-for-sale securities
Yield	4.69%	4.64%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 415,079	$ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27	0
Held-to-maturity securities, gross unrealized losses	(44,164)	(68,398)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 370,942	$ 355,188
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%
Investments	$ 2,516,393	$ 2,542,202
Gross unrealized gains on investments	2,102	2,910
Gross unrealized losses on investments	(129,108)	(194,827)
Total financial assets	$ 2,389,387	$ 2,350,285
Yield on investments	4.39%	4.35%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 1,750	$ 3,501
Amortized cost, 1 to 5 years	16,747	18,894
Amortized cost, 5 to 10 years	110,295	87,922
Amortized cost, over 10 years	103,594	106,340
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	0	0
Gross unrealized gains, 1 to 5 years	8	0
Gross unrealized gains, 5 to 10 years	189	177
Gross unrealized gains, over 10 years	267	831
Gross unrealized losses, Within 1 year	(5)	(36)
Gross unrealized losses, 1 to 5 years	(305)	(563)
Gross unrealized losses, 5 to 10 years	(173)	0
Gross unrealized losses, over 10 years	(8)	(13)
Fair Value, Available-for-sale securities
Fair value, within 1 year	1,745	3,465
Fair value, 1 to 5 years	16,450	18,331
Fair value, 5 to 10 years	110,311	88,099
Fair value, over 10 years	$ 103,853	$ 107,158
Yield, Available-for-sale securities
Yield, within 1 year	6.07%	6.06%
Yield, 1 to 5 years	4.54%	4.70%
Yield, 5 to 10 years	6.01%	5.76%
Yield, over 10 years	6.09%	5.84%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 17,693	$ 18,579
Amortized cost, 5 to 10 years	31,469	36,875
Amortized cost, over 10 years	519,103	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	14	2
Gross unrealized gains, over 10 years	316	578
Gross unrealized losses, 1 to 5 years	(766)	(715)
Gross unrealized losses, 5 to 10 years	(205)	(99)
Gross unrealized losses, over 10 years	(6,320)	(7,115)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	16,927	17,864
Fair value, 5 to 10 years	31,278	36,778
Fair value, over 10 years	$ 513,099	$ 533,374
Yield, Available-for-sale securities
Yield, 1 to 5 years	6.08%	6.06%
Yield, 5 to 10 years	6.23%	6.11%
Yield, over 10 years	6.37%	6.35%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 152,022	$ 151,893
Amortized cost, 5 to 10 years	112,923	113,221
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	479	895
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized losses, 1 to 5 years	(1,196)	(1,787)
Gross unrealized losses, 5 to 10 years	(15,415)	(21,700)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	151,305	151,001
Fair value, 5 to 10 years	$ 97,508	$ 91,521
Yield, Available-for-sale securities
Yield, 1 to 5 years	5.15%	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	$ 5,712	$ 5,720
Amortized cost, over 10 years	29,822	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	95	361
Gross unrealized losses, 5 to 10 years	(298)	(701)
Gross unrealized losses, over 10 years	(125)	(550)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	5,414	5,019
Fair value, over 10 years	$ 29,792	$ 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	$ 1,000,184	$ 1,005,928
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	707	66
Gross unrealized losses, without single maturity date	(60,128)	(93,150)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 940,763	$ 912,844
Yield, Available-for-sale securities
Yield, without single maturity date	3.44%	3.39%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities	$ 415,079	$ 423,586
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27	0
Held-to-maturity securities, gross unrealized losses	(44,164)	(68,398)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 370,942	$ 355,188
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%