Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,667,467	$ 1,458,071
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	44,215	30,461
Gross unrealized loss	(18,635)	(2,790)
Fair Value, Available-for-sale securities
Fair value	$ 1,693,047	$ 1,485,742
Yield, Available-for-sale securities
Yield	2.83%	3.27%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 920,255	$ 1,443,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27,938	19,952
Held-to-maturity securities, gross unrealized losses	0	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 948,193	$ 1,463,095
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.16%	3.15%
Investments	$ 2,587,722	$ 2,901,551
Gross unrealized gains on investments	72,153	50,413
Gross unrealized losses on investments	(18,635)	(3,127)
Total financial assets	$ 2,641,240	$ 2,948,837
Yield on investments	2.95%	3.21%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 61,083	$ 65,287
Amortized cost, over 10 years	286,932	207,067
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	286	39
Gross unrealized gains, over 10 years	0	1
Gross unrealized losses, 5 to 10 years	(2,069)	(629)
Gross unrealized losses, over 10 years	(7,871)	(987)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	59,300	64,697
Fair value, over 10 years	$ 279,061	$ 206,081
Yield, Available-for-sale securities
Yield, 5 to 10 years	1.71%	2.43%
Yield, over 10 years	1.89%	3.02%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 43,973	$ 43,903
Amortized cost, 1 to 5 years	70,000	70,000
Amortized cost, 5 to 10 years	150,205	92,931
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	7	411
Gross unrealized gains, 1 to 5 years	69	689
Gross unrealized gains, 5 to 10 years	1,982	1,879
Gross unrealized losses, Within 1 year	(4)	0
Gross unrealized losses, 1 to 5 years	(3,222)	(50)
Gross unrealized losses, 5 to 10 years	(3,486)	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	43,976	44,314
Fair value, 1 to 5 years	66,847	70,639
Fair value, 5 to 10 years	$ 148,701	$ 94,810
Yield, Available-for-sale securities
Yield, within 1 year	2.34%	3.65%
Yield, 1 to 5 years	2.81%	3.29%
Yield, 5 to 10 years	2.85%	3.27%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 1,445	$ 1,430
Amortized cost, over 10 years	20,293	20,303
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	20	14
Gross unrealized gains, over 10 years	239	895
Gross unrealized losses, 1 to 5 years	0	0
Gross unrealized losses, over 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	1,465	1,444
Fair value, over 10 years	$ 20,532	$ 21,198
Yield, Available-for-sale securities
Yield, 1 to 5 years	1.96%	1.94%
Yield, over 10 years	6.45%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 1,033,536	$ 957,150
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	41,612	26,533
Gross unrealized losses, without single maturity date	(1,983)	(1,124)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 1,073,165	$ 982,559
Yield, Available-for-sale securities
Yield, without single maturity date	3.10%	3.29%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 913,243	$ 1,428,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27,938	19,945
Held-to-maturity securities, gross unrealized losses	0	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 941,181	$ 1,448,088
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.17%	3.15%
Commercial MBS
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 7,012	$ 15,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	0	7
Held-to-maturity securities, gross unrealized losses	0	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 7,012	$ 15,007
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	1.67%	2.89%