Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,431,874	$ 2,210,099
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	54,923	49,007
Gross unrealized loss	(3,853)	(9,614)
Fair Value, Available-for-sale securities
Fair value	$ 2,482,944	$ 2,249,492
Yield, Available-for-sale securities
Yield	1.73%	1.97%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 586,870	$ 705,838
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	19,309	21,582
Held-to-maturity securities, gross unrealized losses	(4)	(52)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 606,175	$ 727,368
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.14%	3.14%
Investments	$ 3,018,744	$ 2,915,937
Gross unrealized gains on investments	74,232	70,589
Gross unrealized losses on investments	(3,857)	(9,666)
Total financial assets	$ 3,089,119	$ 2,976,860
Yield on investments	2.00%	2.25%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 58,807
Amortized cost, 5 to 10 years	18,443	$ 18,448
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	588
Gross unrealized gains, 5 to 10 years	443	376
Gross unrealized losses, 1 to 5 years	0
Gross unrealized losses, 5 to 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	59,395
Fair value, 5 to 10 years	$ 18,886	$ 18,824
Yield, Available-for-sale securities
Yield, 1 to 5 years	0.53%
Yield, 5 to 10 years	2.05%	2.05%
Asset-backed securities
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 107,355	$ 38,289
Amortized cost, over 10 years	1,097,828	906,489
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	206	0
Gross unrealized gains, over 10 years	7,064	647
Gross unrealized losses, 5 to 10 years	(1,360)	(1,600)
Gross unrealized losses, over 10 years	(737)	(6,908)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	106,201	36,689
Fair value, over 10 years	$ 1,104,155	$ 900,228
Yield, Available-for-sale securities
Yield, 5 to 10 years	0.68%	0.83%
Yield, over 10 years	1.09%	1.14%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 52,756	$ 54,209
Amortized cost, 1 to 5 years	152,226	128,289
Amortized cost, 5 to 10 years	73,340	97,157
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	236	337
Gross unrealized gains, 1 to 5 years	6,557	3,366
Gross unrealized gains, 5 to 10 years	2,451	4,305
Gross unrealized losses, Within 1 year	(86)	(51)
Gross unrealized losses, 1 to 5 years	(87)	(428)
Gross unrealized losses, 5 to 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	52,906	54,495
Fair value, 1 to 5 years	158,696	131,227
Fair value, 5 to 10 years	$ 75,791	$ 101,462
Yield, Available-for-sale securities
Yield, within 1 year	1.22%	1.22%
Yield, 1 to 5 years	1.87%	1.78%
Yield, 5 to 10 years	1.16%	1.50%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 1,469	$ 1,461
Amortized cost, over 10 years	35,836	36,044
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	33	36
Gross unrealized gains, over 10 years	1,365	774
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	1,502	1,497
Fair value, over 10 years	$ 37,201	$ 36,818
Yield, Available-for-sale securities
Yield, 1 to 5 years	0.00%	0.00%
Yield, over 10 years	5.40%	5.40%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 833,814	$ 929,713
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	35,980	39,166
Gross unrealized losses, without single maturity date	(1,583)	(627)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 868,211	$ 968,252
Yield, Available-for-sale securities
Yield, without single maturity date	2.72%	2.82%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 581,313	$ 698,934
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	19,309	21,582
Held-to-maturity securities, gross unrealized losses	0	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 600,622	$ 720,516
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.16%	3.16%
Commercial MBS
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 5,557	$ 6,904
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	0	0
Held-to-maturity securities, gross unrealized losses	(4)	(52)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 5,553	$ 6,852
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	1.02%	1.02%