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Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,914,669	$ 2,090,070
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	35,377	50,829
Gross unrealized loss	(3,907)	(2,640)
Fair Value, Available-for-sale securities
Fair value	$ 1,946,139	$ 2,138,259
Yield, Available-for-sale securities
Yield	1.72%	1.69%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 326,387	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	9,508	13,522
Held-to-maturity securities, gross unrealized losses	0	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 335,895	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.17%	3.17%
Investments	$ 2,241,056	$ 2,456,095
Gross unrealized gains on investments	44,885	64,351
Gross unrealized losses on investments	(3,907)	(2,640)
Total financial assets	$ 2,282,034	$ 2,517,806
Yield on investments	1.91%	1.89%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 57,562	$ 47,339
Amortized cost, 5 to 10 years		14,064
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	93	240
Gross unrealized gains, 5 to 10 years		136
Gross unrealized losses, 1 to 5 years	(88)	0
Gross unrealized losses, 5 to 10 years		0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	$ 57,567	47,579
Fair value, 5 to 10 years		$ 14,200
Yield, Available-for-sale securities
Yield, 1 to 5 years	0.86%	0.44%
Yield, 5 to 10 years		2.05%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 19,280	$ 19,730
Amortized cost, 5 to 10 years	66,245	71,207
Amortized cost, over 10 years	867,872	973,892
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	0
Gross unrealized gains, 5 to 10 years	290	412
Gross unrealized gains, over 10 years	7,206	14,069
Gross unrealized losses, 1 to 5 years	(471)	(434)
Gross unrealized losses, 5 to 10 years	(138)	(97)
Gross unrealized losses, over 10 years	(390)	(98)
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	18,809	19,296
Fair value, 5 to 10 years	66,397	71,522
Fair value, over 10 years	$ 874,688	$ 987,863
Yield, Available-for-sale securities
Yield, 1 to 5 years	0.83%	0.52%
Yield, 5 to 10 years	0.58%	0.57%
Yield, over 10 years	0.97%	0.94%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year		$ 225,928
Amortized cost, 1 to 5 years	$ 226,048	86,802
Amortized cost, 5 to 10 years	86,475	28,804
Amortized cost, over 10 years	28,833
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year		7,860
Gross unrealized gains, 1 to 5 years	6,355	1,345
Gross unrealized gains, 5 to 10 years	409	249
Gross unrealized gains, over 10 years	66
Gross unrealized losses, Within 1 year		0
Gross unrealized losses, 1 to 5 years	0	0
Gross unrealized losses, 5 to 10 years	0	0
Gross unrealized losses, over 10 years	0
Fair Value, Available-for-sale securities
Fair value, within 1 year		233,788
Fair value, 1 to 5 years	232,403	88,147
Fair value, 5 to 10 years	86,884	$ 29,053
Fair value, over 10 years	$ 28,899
Yield, Available-for-sale securities
Yield, within 1 year		1.57%
Yield, 1 to 5 years	1.59%	4.64%
Yield, 5 to 10 years	4.64%	1.95%
Yield, over 10 years	1.95%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 1,501	$ 1,493
Amortized cost, 5 to 10 years	5,774	5,781
Amortized cost, over 10 years	29,900	29,909
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	10	17
Gross unrealized gains, 5 to 10 years	226	294
Gross unrealized gains, over 10 years	2,738	2,490
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 5 to 10 years	0	0
Gross unrealized losses, over 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	1,511	1,510
Fair value, 5 to 10 years	6,000	6,075
Fair value, over 10 years	$ 32,638	$ 32,399
Yield, Available-for-sale securities
Yield, within 1 year	0.00%	0.00%
Yield, 5 to 10 years	0.22%	0.22%
Yield, over 10 years	5.85%	5.85%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 525,179	$ 585,121
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	17,984	23,717
Gross unrealized losses, without single maturity date	(2,820)	(2,011)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 540,343	$ 606,827
Yield, Available-for-sale securities
Yield, without single maturity date	2.57%	2.60%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 326,387	$ 366,025
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	9,508	13,522
Held-to-maturity securities, gross unrealized losses	0	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 335,895	$ 379,547
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.17%	3.17%

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