Fair Value Measurements - Reconciliation of Amortized Cost to Fair Value of Securities (Details) - USD ($)
$ in Thousands |
Jun. 30, 2026 |
Sep. 30, 2025 |
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost |
$ 4,229,746
|
$ 3,542,438
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains |
24,293
|
40,997
|
| Gross unrealized loss |
(63,776)
|
(50,234)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value |
$ 4,190,263
|
$ 3,533,201
|
| Yield, Available-for-sale securities |
|
|
| Yield |
4.06%
|
4.17%
|
| Held-to-maturity securities |
|
|
| Held-to-maturity securities, at amortized cost |
$ 858,261
|
$ 645,802
|
| Investments |
5,088,007
|
4,188,240
|
| Gross unrealized gains on investments |
26,660
|
45,070
|
| Gross unrealized losses on investments |
(102,888)
|
(87,370)
|
| Total financial assets |
$ 5,011,779
|
$ 4,145,940
|
| Yield on investments |
4.08%
|
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 |
306
|
545
|
| Amortized cost, 5 to 10 years |
127,047
|
158,026
|
| Amortized cost, over 10 years |
62,237
|
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 |
175
|
162
|
| Gross unrealized gains, over 10 years |
208
|
163
|
| Gross unrealized losses, Within 1 year |
(5)
|
(29)
|
| Gross unrealized losses, 1 to 5 years |
(1)
|
0
|
| Gross unrealized losses, 5 to 10 years |
(86)
|
(295)
|
| Gross unrealized losses, over 10 years |
(113)
|
(227)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, within 1 year |
1,124
|
1,658
|
| Fair value, 1 to 5 years |
305
|
545
|
| Fair value, 5 to 10 years |
127,136
|
157,893
|
| Fair value, over 10 years |
$ 62,332
|
$ 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 |
$ 9,445
|
$ 10,492
|
| Amortized cost, 5 to 10 years |
|
3,945
|
| Amortized cost, over 10 years |
416,906
|
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 |
|
7
|
| Gross unrealized gains, over 10 years |
855
|
1,702
|
| Gross unrealized losses, Within 1 year |
(143)
|
(163)
|
| Gross unrealized losses, 5 to 10 years |
|
0
|
| Gross unrealized losses, over 10 years |
(939)
|
(1,383)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, within 1 year |
9,302
|
10,329
|
| Fair value, 5 to 10 years |
|
3,952
|
| Fair value, over 10 years |
$ 416,822
|
$ 492,053
|
| Yield, Available-for-sale securities |
|
|
| Yield, within 1 year |
4.39%
|
5.12%
|
| Yield, 5 to 10 years |
|
5.27%
|
| Yield, over 10 years |
4.69%
|
5.36%
|
| Corporate debt securities |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, 1 to 5 years |
$ 90,572
|
$ 32,821
|
| Amortized cost, 5 to 10 years |
79,352
|
128,015
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, 1 to 5 years |
464
|
0
|
| Gross unrealized gains, 5 to 10 years |
0
|
314
|
| Gross unrealized losses, 1 to 5 years |
(5,910)
|
(2,047)
|
| Gross unrealized losses, 5 to 10 years |
(3,915)
|
(6,566)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, 1 to 5 years |
85,126
|
30,774
|
| Fair value, 5 to 10 years |
$ 75,437
|
$ 121,763
|
| Yield, Available-for-sale securities |
|
|
| Yield, 1 to 5 years |
4.90%
|
4.95%
|
| Yield, 5 to 10 years |
3.94%
|
4.37%
|
| Municipal bonds due |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, 1 to 5 years |
$ 5,598
|
|
| Amortized cost, 5 to 10 years |
20,000
|
$ 25,659
|
| Amortized cost, over 10 years |
9,698
|
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 |
290
|
250
|
| Gross unrealized gains, over 10 years |
0
|
0
|
| Gross unrealized losses, 1 to 5 years |
(96)
|
|
| Gross unrealized losses, 5 to 10 years |
0
|
(179)
|
| Gross unrealized losses, over 10 years |
(214)
|
(226)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, 1 to 5 years |
5,502
|
|
| Fair value, 5 to 10 years |
20,290
|
25,730
|
| Fair value, over 10 years |
$ 9,484
|
$ 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 |
4.59%
|
4.57%
|
| Agency pass-through certificates |
|
|
| Amortized Cost, Available-for-sale securities |
|
|
| Amortized cost, without single maturity date |
$ 3,407,456
|
$ 2,603,873
|
| Gross Unrealized Gains / Losses, Available-for-sale securities |
|
|
| Gross unrealized gains, without single maturity date |
22,301
|
38,399
|
| Gross unrealized losses, without single maturity date |
(52,354)
|
(39,119)
|
| Fair Value, Available-for-sale securities |
|
|
| Fair value, without single maturity date |
$ 3,377,403
|
$ 2,603,153
|
| Yield, Available-for-sale securities |
|
|
| Yield, without single maturity date |
3.93%
|
3.83%
|
| Held-to-maturity securities |
|
|
| Held-to-maturity securities, at amortized cost |
$ 858,261
|
$ 645,802
|
| Held-to-maturity securities, gross unrealized gains |
2,367
|
4,073
|
| Held-to-maturity securities, gross unrealized losses |
(39,112)
|
(37,136)
|
| Held-to-maturity securities, fair value |
$ 821,516
|
$ 612,739
|
| Held-to-maturity securities, yield |
4.22%
|
3.85%
|