Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Sep. 30, 2022
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,100,465	$ 2,162,737
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	2,803	1,308
Gross unrealized loss	(96,982)	(113,008)
Fair Value, Available-for-sale securities
Fair value	$ 2,006,286	$ 2,051,037
Yield, Available-for-sale securities
Yield	4.18%	3.36%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 445,222	$ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	55	22
Held-to-maturity securities, gross unrealized losses	(44,201)	(56,461)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 401,076	$ 406,860
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%
Investments	$ 2,545,687	$ 2,626,036
Gross unrealized gains on investments	2,858	1,330
Gross unrealized losses on investments	(141,183)	(169,469)
Total financial assets	$ 2,407,362	$ 2,457,897
Yield on investments	3.97%	3.28%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 7,003
Amortized cost, 1 to 5 years	24,228	$ 40,403
Amortized cost, over 10 years	63,740
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	0
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, over 10 years	444
Gross unrealized losses, Within 1 year	(39)
Gross unrealized losses, 1 to 5 years	(604)	(1,049)
Gross unrealized losses, over 10 years	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	6,964
Fair value, 1 to 5 years	23,624	$ 39,354
Fair value, over 10 years	$ 64,184
Yield, Available-for-sale securities
Yield, within 1 year	5.29%
Yield, 1 to 5 years	4.29%	3.03%
Yield, over 10 years	5.15%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 20,059	$ 22,527
Amortized cost, 5 to 10 years	49,023	82,962
Amortized cost, over 10 years	600,080	668,482
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	4	4
Gross unrealized gains, over 10 years	753	783
Gross unrealized losses, 1 to 5 years	(763)	(1,141)
Gross unrealized losses, 5 to 10 years	(280)	(547)
Gross unrealized losses, over 10 years	(14,806)	(6,069)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	19,296	21,386
Fair value, 5 to 10 years	48,747	82,419
Fair value, over 10 years	$ 586,027	$ 663,196
Yield, Available-for-sale securities
Yield, 1 to 5 years	5.48%	3.27%
Yield, 5 to 10 years	5.51%	3.53%
Yield, over 10 years	5.52%	3.86%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 75,023	$ 75,000
Amortized cost, 1 to 5 years	151,652	151,411
Amortized cost, 5 to 10 years	113,817	114,414
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	2	4
Gross unrealized gains, 1 to 5 years	793	0
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized losses, Within 1 year	(107)	(200)
Gross unrealized losses, 1 to 5 years	(1,683)	(2,748)
Gross unrealized losses, 5 to 10 years	(17,094)	(24,124)
Fair Value, Available-for-sale securities
Fair value, within 1 year	74,918	74,804
Fair value, 1 to 5 years	150,762	148,663
Fair value, 5 to 10 years	$ 96,723	$ 90,290
Yield, Available-for-sale securities
Yield, within 1 year	5.36%	3.74%
Yield, 1 to 5 years	4.75%	3.59%
Yield, 5 to 10 years	3.87%	3.87%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 5,735	$ 5,751
Amortized cost, over 10 years	29,851	29,871
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, over 10 years	275	400
Gross unrealized losses, 5 to 10 years	(201)	(361)
Gross unrealized losses, over 10 years	(363)	(699)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	5,534	5,390
Fair value, over 10 years	$ 29,763	$ 29,572
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	$ 960,254	$ 971,916
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	532	117
Gross unrealized losses, without single maturity date	(61,042)	(76,070)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 899,744	$ 895,963
Yield, Available-for-sale securities
Yield, without single maturity date	3.02%	2.81%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 445,222	$ 463,299
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	55	22
Held-to-maturity securities, gross unrealized losses	(44,201)	(56,461)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 401,076	$ 406,860
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.88%	2.88%