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Investment Securities (Tables) | 12 Months Ended
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Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract] |
Schedule of Investments by Contractual Maturity Date | The tables below provide detail regarding the amortized cost and fair value of available-for-sale and held-to-maturity investment securities.
September 30, 2023 | Amortized Cost |  | Gross Unrealized |  | Fair Value |  | Yield
Gains |  | Losses |
| ($ in thousands)
Available-for-sale securities |  |  |  |  |  |  |  |  |
U.S. government and agency securities due |  |  |  |  |  |  |  |  |
Within 1 year | $ | 3,501 |  |  | $ | — |  |  | $ | (36) |  |  | $ | 3,465 |  |  | 6.06 | %
1 to 5 years | 18,894 |  |  | — |  |  | (563) |  |  | 18,331 |  |  | 4.70 |
5 to 10 years | 87,922 |  |  | 177 |  |  | — |  |  | 88,099 |  |  | 5.76 |
Over 10 years | 106,340 |  |  | 831 |  |  | (13) |  |  | 107,158 |  |  | 5.84 |
Asset-backed securities due |  |  |  |  |  |  |  |  |
1 to 5 years | 18,579 |  |  | — |  |  | (715) |  |  | 17,864 |  |  | 6.06 |
5 to 10 years | 36,875 |  |  | 2 |  |  | (99) |  |  | 36,778 |  |  | 6.11 |
Over 10 years | 539,911 |  |  | 578 |  |  | (7,115) |  |  | 533,374 |  |  | 6.35 |
Corporate debt securities due |  |  |  |  |  |  |  |  |
1 to 5 years | 151,893 |  |  | 895 |  |  | (1,787) |  |  | 151,001 |  |  | 5.14 |
5 to 10 years | 113,221 |  |  | — |  |  | (21,700) |  |  | 91,521 |  |  | 3.87 |
Municipal bonds due |  |  |  |  |  |  |  |  |
5 to 10 years | 5,720 |  |  | — |  |  | (701) |  |  | 5,019 |  |  | 3.00 |
Over 10 years | 29,832 |  |  | 361 |  |  | (550) |  |  | 29,643 |  |  | 5.85 |
Mortgage-backed securities |  |  |  |  |  |  |  |  |
Agency pass-through certificates | 1,005,928 |  |  | 66 |  |  | (93,150) |  |  | 912,844 |  |  | 3.39 |
| 2,118,616 |  |  | 2,910 |  |  | (126,429) |  |  | 1,995,097 |  |  | 4.64 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |
Mortgage-backed securities |  |  |  |  |  |  |  |  |
Agency pass-through certificates | 423,586 |  |  | — |  |  | (68,398) |  |  | 355,188 |  |  | 2.88 |
| 423,586 |  |  | — |  |  | (68,398) |  |  | 355,188 |  |  | 2.88 |
| $ | 2,542,202 |  |  | $ | 2,910 |  |  | $ | (194,827) |  |  | $ | 2,350,285 |  |  | 4.35 | %
September 30, 2022 | Amortized Cost |  | Gross Unrealized |  | Fair Value |  | Yield
Gains |  | Losses |
| ($ in thousands)
Available-for-sale securities |  |  |  |  |  |  |  |  |
U.S. government and agency securities due |  |  |  |  |  |  |  |  |
1 to 5 years | $ | 40,403 |  |  | $ | — |  |  | $ | (1,049) |  |  | $ | 39,354 |  |  | 3.03 | %
Asset-backed securities due |  |  |  |  |  |  |  |  |
1 to 5 years | 22,527 |  |  | — |  |  | (1,141) |  |  | 21,386 |  |  | 3.27 |
5 to 10 years | 82,962 |  |  | 4 |  |  | (547) |  |  | 82,419 |  |  | 3.53 |
Over 10 years | 668,482 |  |  | 783 |  |  | (6,069) |  |  | 663,196 |  |  | 3.86 |
Corporate debt securities due |  |  |  |  |  |  |  |  |
Within 1 year | 75,000 |  |  | 4 |  |  | (200) |  |  | 74,804 |  |  | 3.74 |
1 to 5 years | 151,411 |  |  | — |  |  | (2,748) |  |  | 148,663 |  |  | 3.59 |
5 to 10 years | 114,414 |  |  | — |  |  | (24,124) |  |  | 90,290 |  |  | 3.87 |
Municipal bonds due |  |  |  |  |  |  |  |  |
5 to 10 years | 5,751 |  |  | — |  |  | (361) |  |  | 5,390 |  |  | 3.00 |
Over 10 years | 29,871 |  |  | 400 |  |  | (699,000) |  |  | 29,572 |  |  | 5.85 |
Mortgage-backed securities |  |  |  |  |  |  |  |  |
Agency pass-through certificates | 971,916 |  |  | 117 |  |  | (76,070) |  |  | 895,963 |  |  | 2.81 |
| 2,162,737 |  |  | 1,308 |  |  | (113,008) |  |  | 2,051,037 |  |  | 3.36 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |
Mortgage-backed securities |  |  |  |  |  |  |  |  |
Agency pass-through certificates | 463,299 |  |  | 22 |  |  | (56,461) |  |  | 406,860 |  |  | 2.88 |
| 463,299 |  |  | 22 |  |  | (56,461) |  |  | 406,860 |  |  | 2.88 |
| $ | 2,626,036 |  |  | $ | 1,330 |  |  | $ | (169,469) |  |  | $ | 2,457,897 |  |  | 3.28 | %
Schedule of Unrealized Losses and Fair Value of Securities | The following tables show the gross unrealized losses and fair value of securities as of September 30, 2023 and September 30, 2022, by length of time that individual securities in each category have been in a continuous loss position. There were 232 and 223 securities with an unrealized loss as of September 30, 2023 and September 30, 2022, respectively. The decline in fair value since purchase is attributable to changes in interest rates. Because the Company does not intend to sell these securities and does not consider it more likely than not that it will be required to sell these securities before the recovery of amortized cost basis, which may be upon maturity, the Company does not consider these investments to be impaired.
September 30, 2023 | Less than 12 months |  | 12 months or more |  | Total
| Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value
| (In thousands) |  |
Available-for-sale securities |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities | $ | — |  |  | $ | — |  |  | $ | (23,487) |  |  | $ | 167,452 |  |  | $ | (23,487) |  |  | $ | 167,452 |
Municipal bonds | — |  |  | — |  |  | (1,250) |  |  | 14,302 |  |  | (1,250) |  |  | 14,302 |
U.S. government and agency securities | (13) |  |  | 14,917 |  |  | (599) |  |  | 21,795 |  |  | (612) |  |  | 36,712 |
Asset-backed securities | (2,142) |  |  | 86,800 |  |  | (5,788) |  |  | 445,454 |  |  | (7,930) |  |  | 532,254 |
Mortgage-backed securities | (2,030) |  |  | 142,235 |  |  | (91,120) |  |  | 744,010 |  |  | (93,150) |  |  | 886,245 |
| (4,185) |  |  | 243,952 |  |  | (122,244) |  |  | 1,393,013 |  |  | (126,429) |  |  | 1,636,965 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |  |  |
Mortgage-backed securities | (15) |  |  | 1,424 |  |  | (68,383) |  |  | 353,764 |  |  | (68,398) |  |  | 355,188 |
| $ | (4,200) |  |  | $ | 245,376 |  |  | $ | (190,627) |  |  | $ | 1,746,777 |  |  | $ | (194,827) |  |  | $ | 1,992,153 |
September 30, 2022 | Less than 12 months |  | 12 months or more |  | Total
| Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value
| (In thousands) |  |
Available-for-sale securities |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities | $ | (27,072) |  |  | $ | 288,753 |  |  | $ | — |  |  | $ | — |  |  | $ | (27,072) |  |  | $ | 288,753 |
Municipal bonds due | (1,061) |  |  | 14,561 |  |  | — |  |  | — |  |  | (1,061) |  |  | 14,561 |
U.S. government and agency securities | (1,049) |  |  | 39,354 |  |  | — |  |  | — |  |  | (1,049) |  |  | 39,354 |
Asset-backed securities | (6,374) |  |  | 601,248 |  |  | (1,383) |  |  | 50,070 |  |  | (7,757) |  |  | 651,318 |
Mortgage-backed securities | (63,738) |  |  | 833,683 |  |  | (12,331) |  |  | 53,533 |  |  | (76,069) |  |  | 887,216 |
| (99,294) |  |  | 1,777,599 |  |  | (13,714) |  |  | 103,603 |  |  | (113,008) |  |  | 1,881,202 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |  |  |
Mortgage-backed securities | (56,461) |  |  | 405,166 |  |  | — |  |  | — |  |  | (56,461) |  |  | 405,166 |
| $ | (155,755) |  |  | $ | 2,182,765 |  |  | $ | (13,714) |  |  | $ | 103,603 |  |  | $ | (169,469) |  |  | $ | 2,286,368 |
Schedule of Debt Securities, Held-to-Maturity, Continuous Unrealized Loss Position, Fair Value | The following tables show the gross unrealized losses and fair value of securities as of September 30, 2023 and September 30, 2022, by length of time that individual securities in each category have been in a continuous loss position. There were 232 and 223 securities with an unrealized loss as of September 30, 2023 and September 30, 2022, respectively. The decline in fair value since purchase is attributable to changes in interest rates. Because the Company does not intend to sell these securities and does not consider it more likely than not that it will be required to sell these securities before the recovery of amortized cost basis, which may be upon maturity, the Company does not consider these investments to be impaired.
September 30, 2023 | Less than 12 months |  | 12 months or more |  | Total
| Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value
| (In thousands) |  |
Available-for-sale securities |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities | $ | — |  |  | $ | — |  |  | $ | (23,487) |  |  | $ | 167,452 |  |  | $ | (23,487) |  |  | $ | 167,452 |
Municipal bonds | — |  |  | — |  |  | (1,250) |  |  | 14,302 |  |  | (1,250) |  |  | 14,302 |
U.S. government and agency securities | (13) |  |  | 14,917 |  |  | (599) |  |  | 21,795 |  |  | (612) |  |  | 36,712 |
Asset-backed securities | (2,142) |  |  | 86,800 |  |  | (5,788) |  |  | 445,454 |  |  | (7,930) |  |  | 532,254 |
Mortgage-backed securities | (2,030) |  |  | 142,235 |  |  | (91,120) |  |  | 744,010 |  |  | (93,150) |  |  | 886,245 |
| (4,185) |  |  | 243,952 |  |  | (122,244) |  |  | 1,393,013 |  |  | (126,429) |  |  | 1,636,965 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |  |  |
Mortgage-backed securities | (15) |  |  | 1,424 |  |  | (68,383) |  |  | 353,764 |  |  | (68,398) |  |  | 355,188 |
| $ | (4,200) |  |  | $ | 245,376 |  |  | $ | (190,627) |  |  | $ | 1,746,777 |  |  | $ | (194,827) |  |  | $ | 1,992,153 |
September 30, 2022 | Less than 12 months |  | 12 months or more |  | Total
| Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value |  | Unrealized Gross Losses |  | Fair Value
| (In thousands) |  |
Available-for-sale securities |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities | $ | (27,072) |  |  | $ | 288,753 |  |  | $ | — |  |  | $ | — |  |  | $ | (27,072) |  |  | $ | 288,753 |
Municipal bonds due | (1,061) |  |  | 14,561 |  |  | — |  |  | — |  |  | (1,061) |  |  | 14,561 |
U.S. government and agency securities | (1,049) |  |  | 39,354 |  |  | — |  |  | — |  |  | (1,049) |  |  | 39,354 |
Asset-backed securities | (6,374) |  |  | 601,248 |  |  | (1,383) |  |  | 50,070 |  |  | (7,757) |  |  | 651,318 |
Mortgage-backed securities | (63,738) |  |  | 833,683 |  |  | (12,331) |  |  | 53,533 |  |  | (76,069) |  |  | 887,216 |
| (99,294) |  |  | 1,777,599 |  |  | (13,714) |  |  | 103,603 |  |  | (113,008) |  |  | 1,881,202 |
Held-to-maturity securities |  |  |  |  |  |  |  |  |  |  |
Mortgage-backed securities | (56,461) |  |  | 405,166 |  |  | — |  |  | — |  |  | (56,461) |  |  | 405,166 |
| $ | (155,755) |  |  | $ | 2,182,765 |  |  | $ | (13,714) |  |  | $ | 103,603 |  |  | $ | (169,469) |  |  | $ | 2,286,368 |

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