Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,022,414	$ 1,458,071
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	49,713	30,461
Gross unrealized loss	(8,167)	(2,790)
Fair Value, Available-for-sale securities
Fair value	$ 2,063,960	$ 1,485,742
Yield, Available-for-sale securities
Yield	2.20%	3.27%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 827,316	$ 1,443,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27,206	19,952
Held-to-maturity securities, gross unrealized losses	(105)	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 854,417	$ 1,463,095
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.14%	3.15%
Investments	$ 2,849,730	$ 2,901,551
Gross unrealized gains on investments	76,919	50,413
Gross unrealized losses on investments	(8,272)	(3,127)
Total financial assets	$ 2,918,377	$ 2,948,837
Yield on investments	2.47%	3.21%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 59,799	$ 65,287
Amortized cost, over 10 years	616,095	207,067
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	416	39
Gross unrealized gains, over 10 years	74	1
Gross unrealized losses, 5 to 10 years	(2,203)	(629)
Gross unrealized losses, over 10 years	(4,276)	(987)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	58,012	64,697
Fair value, over 10 years	$ 611,893	$ 206,081
Yield, Available-for-sale securities
Yield, 5 to 10 years	1.26%	2.43%
Yield, over 10 years	1.18%	3.02%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 71,064	$ 43,903
Amortized cost, 1 to 5 years	128,325	70,000
Amortized cost, 5 to 10 years	117,000	92,931
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	66	411
Gross unrealized gains, 1 to 5 years	3,256	689
Gross unrealized gains, 5 to 10 years	3,201	1,879
Gross unrealized losses, Within 1 year	(63)	0
Gross unrealized losses, 1 to 5 years	(849)	(50)
Gross unrealized losses, 5 to 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	71,067	44,314
Fair value, 1 to 5 years	130,732	70,639
Fair value, 5 to 10 years	$ 120,201	$ 94,810
Yield, Available-for-sale securities
Yield, within 1 year	0.68%	3.65%
Yield, 1 to 5 years	2.04%	3.29%
Yield, 5 to 10 years	1.70%	3.27%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 1,453	$ 1,430
Amortized cost, over 10 years	36,066	20,303
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	41	14
Gross unrealized gains, over 10 years	534	895
Gross unrealized losses, 1 to 5 years	0	0
Gross unrealized losses, over 10 years	(53)	0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	1,494	1,444
Fair value, over 10 years	$ 36,547	$ 21,198
Yield, Available-for-sale securities
Yield, 1 to 5 years	0.00%	1.94%
Yield, over 10 years	5.40%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 992,612	$ 957,150
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	42,125	26,533
Gross unrealized losses, without single maturity date	(723)	(1,124)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 1,034,014	$ 982,559
Yield, Available-for-sale securities
Yield, without single maturity date	2.96%	3.29%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 820,304	$ 1,428,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	27,206	19,945
Held-to-maturity securities, gross unrealized losses	0	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 847,510	$ 1,448,088
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.16%	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	(105)	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 6,907	$ 15,007
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	1.06%	2.89%