Fair Value Measurements - Reconciliation of Amortized Cost to Fair Value of Securities (Details) - USD ($)
$ in Thousands |
Mar. 31, 2026 |
Sep. 30, 2025 |
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost |
$ 4,369,073
|
$ 3,542,438
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains |
39,335
|
40,997
|
| Gross unrealized loss |
(56,150)
|
(50,234)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value |
$ 4,352,258
|
$ 3,533,201
|
| Yield, Available-for-sale securities |
|
|
| Yield |
4.00%
|
4.17%
|
| Held-to-maturity securities |
|
|
| Held-to-maturity securities, at amortized cost |
$ 745,727
|
$ 645,802
|
| Investments |
5,114,800
|
4,188,240
|
| Gross unrealized gains on investments |
42,348
|
45,070
|
| Gross unrealized losses on investments |
(92,773)
|
(87,370)
|
| Total financial assets |
$ 5,064,375
|
$ 4,145,940
|
| Yield on investments |
4.01%
|
4.12%
|
| U.S. government and agency securities |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, within 1 year |
$ 1,129
|
$ 1,687
|
| Amortized cost, 1 to 5 years |
347
|
545
|
| Amortized cost, 5 to 10 years |
135,640
|
158,026
|
| Amortized cost, over 10 years |
65,241
|
75,887
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, within 1 year |
0
|
0
|
| Gross unrealized gains, 1 to 5 years |
0
|
0
|
| Gross unrealized gains, 5 to 10 years |
301
|
162
|
| Gross unrealized gains, over 10 years |
224
|
163
|
| Gross unrealized losses, Within 1 year |
(11)
|
(29)
|
| Gross unrealized losses, 1 to 5 years |
0
|
0
|
| Gross unrealized losses, 5 to 10 years |
(4)
|
(295)
|
| Gross unrealized losses, over 10 years |
(140)
|
(227)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, within 1 year |
1,118
|
1,658
|
| Fair value, 1 to 5 years |
347
|
545
|
| Fair value, 5 to 10 years |
135,937
|
157,893
|
| Fair value, over 10 years |
$ 65,325
|
$ 75,823
|
| Yield, Available-for-sale securities |
|
|
| Yield, within 1 year |
2.05%
|
2.05%
|
| Yield, 1 to 5 years |
4.25%
|
5.00%
|
| Yield, 5 to 10 years |
4.20%
|
4.95%
|
| Yield, over 10 years |
4.80%
|
5.52%
|
| Asset-backed securities |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, within 1 year |
$ 9,920
|
$ 10,492
|
| Amortized cost, 5 to 10 years |
1,287
|
3,945
|
| Amortized cost, over 10 years |
464,284
|
491,734
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, within 1 year |
0
|
0
|
| Gross unrealized gains, 5 to 10 years |
0
|
7
|
| Gross unrealized gains, over 10 years |
1,081
|
1,702
|
| Gross unrealized losses, Within 1 year |
(157)
|
(163)
|
| Gross unrealized losses, 5 to 10 years |
(1)
|
0
|
| Gross unrealized losses, over 10 years |
(2,049)
|
(1,383)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, within 1 year |
9,763
|
10,329
|
| Fair value, 5 to 10 years |
1,286
|
3,952
|
| Fair value, over 10 years |
$ 463,316
|
$ 492,053
|
| Yield, Available-for-sale securities |
|
|
| Yield, within 1 year |
4.43%
|
5.12%
|
| Yield, 5 to 10 years |
4.58%
|
5.27%
|
| Yield, over 10 years |
4.67%
|
5.36%
|
| Corporate debt securities |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, 1 to 5 years |
$ 42,508
|
$ 32,821
|
| Amortized cost, 5 to 10 years |
127,732
|
128,015
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, 1 to 5 years |
287
|
0
|
| Gross unrealized gains, 5 to 10 years |
449
|
314
|
| Gross unrealized losses, 1 to 5 years |
(2,142)
|
(2,047)
|
| Gross unrealized losses, 5 to 10 years |
(6,950)
|
(6,566)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, 1 to 5 years |
40,653
|
30,774
|
| Fair value, 5 to 10 years |
$ 121,231
|
$ 121,763
|
| Yield, Available-for-sale securities |
|
|
| Yield, 1 to 5 years |
5.43%
|
4.95%
|
| Yield, 5 to 10 years |
4.14%
|
4.37%
|
| Municipal bonds due |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, 1 to 5 years |
$ 5,643
|
|
| Amortized cost, 5 to 10 years |
20,000
|
$ 25,659
|
| Amortized cost, over 10 years |
9,735
|
9,754
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, 1 to 5 years |
0
|
|
| Gross unrealized gains, 5 to 10 years |
0
|
250
|
| Gross unrealized gains, over 10 years |
0
|
0
|
| Gross unrealized losses, 1 to 5 years |
(208)
|
|
| Gross unrealized losses, 5 to 10 years |
(100)
|
(179)
|
| Gross unrealized losses, over 10 years |
(327)
|
(226)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, 1 to 5 years |
5,435
|
|
| Fair value, 5 to 10 years |
19,900
|
25,730
|
| Fair value, over 10 years |
$ 9,408
|
$ 9,528
|
| Yield, Available-for-sale securities |
|
|
| Yield, 1 to 5 years |
3.00%
|
|
| Yield, 5 to 10 years |
6.45%
|
5.71%
|
| Yield, over 10 years |
4.57%
|
4.57%
|
| Agency pass-through certificates |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, without single maturity date |
$ 3,485,607
|
$ 2,603,873
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, without single maturity date |
36,993
|
38,399
|
| Gross unrealized losses, without single maturity date |
(44,061)
|
(39,119)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, without single maturity date |
$ 3,478,539
|
$ 2,603,153
|
| Yield, Available-for-sale securities |
|
|
| Yield, without single maturity date |
3.86%
|
3.83%
|
| Held-to-maturity securities |
|
|
| Held-to-maturity securities, at amortized cost |
$ 745,727
|
$ 645,802
|
| Held-to-maturity securities, gross unrealized gains |
3,013
|
4,073
|
| Held-to-maturity securities, gross unrealized losses |
(36,623)
|
(37,136)
|
| Held-to-maturity securities, fair value |
$ 712,117
|
$ 612,739
|
| Held-to-maturity securities, yield |
4.04%
|
3.85%
|