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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Sep. 30, 2024
Amortized Cost, Available-for-sale securities    
Amortized cost $ 2,812,767 $ 2,616,877
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains 7,740 10,531
Gross unrealized loss (76,776) (54,699)
Fair Value, Available-for-sale securities    
Fair value $ 2,743,731 $ 2,572,709
Yield, Available-for-sale securities    
Yield 4.67% 4.87%
Amortized Cost, Held-to-maturity securities    
Held-to-maturity securities $ 537,348 $ 436,972
Gross Unrealized Gains / Losses, Held-to-maturity securities    
Held-to-maturity securities, gross unrealized gains 13 913
Held-to-maturity securities, gross unrealized losses (52,255) (36,839)
Fair Value, Held-to-maturity securities    
Held-to-maturity securities, fair value $ 485,106 $ 401,046
Yield, Held-to-maturity securities    
Held-to-maturity securities, yield 3.57% 3.18%
Investments $ 3,350,115 $ 3,053,849
Gross unrealized gains on investments 7,753 11,444
Gross unrealized losses on investments (129,031) (91,538)
Total financial assets $ 3,228,837 $ 2,973,755
Yield on investments 4.41% 4.63%
U.S. government and agency securities    
Amortized Cost, Available-for-sale securities    
Amortized cost, within 1 year $ 2,184 $ 4,360
Amortized cost, 1 to 5 years 4,583 4,640
Amortized cost, 5 to 10 years 195,082 166,070
Amortized cost, over 10 years 90,460 137,799
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, within 1 year 1 4
Gross unrealized gains, 1 to 5 years 1 2
Gross unrealized gains, 5 to 10 years 1,327 1,230
Gross unrealized gains, over 10 years 679 394
Gross unrealized losses, Within 1 year (1) 0
Gross unrealized losses, 1 to 5 years (119) (124)
Gross unrealized losses, 5 to 10 years (4) 0
Gross unrealized losses, over 10 years 0 (171)
Fair Value, Available-for-sale securities    
Fair value, within 1 year 2,184 4,364
Fair value, 1 to 5 years 4,465 4,518
Fair value, 5 to 10 years 196,405 167,300
Fair value, over 10 years $ 91,139 $ 138,022
Yield, Available-for-sale securities    
Yield, within 1 year 5.02% 5.58%
Yield, 1 to 5 years 2.69% 2.82%
Yield, 5 to 10 years 5.45% 5.97%
Yield, over 10 years 5.98% 6.29%
Asset-backed securities    
Amortized Cost, Available-for-sale securities    
Amortized cost, 1 to 5 years $ 11,047 $ 11,466
Amortized cost, 5 to 10 years 8,026 9,631
Amortized cost, over 10 years 528,444 520,756
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, 1 to 5 years 0 0
Gross unrealized gains, 5 to 10 years 13 0
Gross unrealized gains, over 10 years 1,201 600
Gross unrealized losses, 1 to 5 years (264) (284)
Gross unrealized losses, 5 to 10 years 0 (3)
Gross unrealized losses, over 10 years (1,685) (2,041)
Fair Value, Available-for-sale securities    
Fair value, 1 to 5 years 10,783 11,182
Fair value, 5 to 10 years 8,039 9,628
Fair value, over 10 years $ 527,960 $ 519,315
Yield, Available-for-sale securities    
Yield, 1 to 5 years 5.33% 6.04%
Yield, 5 to 10 years 5.48% 6.20%
Yield, over 10 years 5.60% 6.15%
Corporate debt securities    
Amortized Cost, Available-for-sale securities    
Amortized cost, within 1 year $ 45,011 $ 45,024
Amortized cost, 1 to 5 years 74,556 99,244
Amortized cost, 5 to 10 years 161,731 112,029
Amortized cost, over 10 years   50,000
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, within 1 year 0 0
Gross unrealized gains, 1 to 5 years 507 977
Gross unrealized gains, 5 to 10 years 0 0
Gross unrealized gains, over 10 years   0
Gross unrealized losses, Within 1 year (217) (367)
Gross unrealized losses, 1 to 5 years 0 0
Gross unrealized losses, 5 to 10 years (13,714) (10,625)
Gross unrealized losses, over 10 years   0
Fair Value, Available-for-sale securities    
Fair value, within 1 year 44,794 44,657
Fair value, 1 to 5 years 75,063 100,221
Fair value, 5 to 10 years $ 148,017 101,404
Fair value, over 10 years   $ 50,000
Yield, Available-for-sale securities    
Yield, within 1 year 4.61% 4.61%
Yield, 1 to 5 years 5.74% 5.39%
Yield, 5 to 10 years 4.57% 3.87%
Yield, over 10 years   6.85%
Municipal bonds due    
Amortized Cost, Available-for-sale securities    
Amortized cost, 5 to 10 years $ 5,682 $ 5,689
Amortized cost, over 10 years 29,783 29,793
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, 5 to 10 years 0 0
Gross unrealized gains, over 10 years 17 0
Gross unrealized losses, 5 to 10 years (375) (243)
Gross unrealized losses, over 10 years (314) (166)
Fair Value, Available-for-sale securities    
Fair value, 5 to 10 years 5,307 5,446
Fair value, over 10 years $ 29,486 $ 29,627
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 $ 1,656,178 $ 1,420,376
Gross Unrealized Gains / Losses, Available-for-sale securities    
Gross unrealized gains, without single maturity date 3,994 7,324
Gross unrealized losses, without single maturity date (60,083) (40,675)
Fair Value, Available-for-sale securities    
Fair value, without single maturity date $ 1,600,089 $ 1,387,025
Yield, Available-for-sale securities    
Yield, without single maturity date 4.15% 4.09%
Amortized Cost, Held-to-maturity securities    
Held-to-maturity securities $ 537,348 $ 436,972
Gross Unrealized Gains / Losses, Held-to-maturity securities    
Held-to-maturity securities, gross unrealized gains 13 913
Held-to-maturity securities, gross unrealized losses (52,255) (36,839)
Fair Value, Held-to-maturity securities    
Held-to-maturity securities, fair value $ 485,106 $ 401,046
Yield, Held-to-maturity securities    
Held-to-maturity securities, yield 3.57% 3.18%