Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,263,960	$ 1,909,184
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	13,368	25,422
Gross unrealized loss	(6,914)	(11,712)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 1,270,414	$ 1,922,894
Yield, Available-for-sale securities
Yield	2.77%	2.22%
Yield, Held-to-maturity securities
Investments	$ 2,915,488	$ 3,326,783
Gross unrealized gains on investments	18,456	49,593
Gross unrealized losses on investments	(32,785)	(11,926)
Total financial assets	$ 2,901,159	$ 3,364,450
Yield on investments	2.97%	2.62%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 9,300	$ 21,284
1 to 5 years	9,933	12,477
5 to 10 years	55,202	48,134
Over 10 years	149,205	182,051
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	352	0
Gross unrealized gains, 1 to 5 years	3	1,027
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, Over 10 years	107	27
Gross unrealized losses, Within 1 year	0	(59)
Gross unrealized losses, 1 to 5 years	0	(11)
Gross unrealized losses, 5 to 10 years	(944)	(1,589)
Gross unrealized losses, Over 10 years	(913)	(3,990)
Fair Value, Available-for-sale securities
Within 1 year	9,652	21,225
1 to 5 years	9,936	13,493
5 to 10 years	54,258	46,545
Over 10 years	$ 148,399	$ 178,088
Yield, Available-for-sale securities
Within 1 year	10.38%	0.81%
1 to 5 years	1.46%	7.94%
5 to 10 years	1.95%	1.14%
Over 10 years	1.89%	1.33%
Equity securities
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 500	$ 100,422
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	22	1,402
Gross unrealized losses, 1 to 5 years	0	0
Fair Value, Available-for-sale securities
1 to 5 years	$ 522	$ 101,824
Yield, Available-for-sale securities
1 to 5 years	1.80%	1.90%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 28,094	$ 278,094
1 to 5 years	63,587	63,481
5 to 10 years	119,959	69,955
Over 10 years		50,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	18	325
Gross unrealized gains, 1 to 5 years	1,621	928
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, Over 10 years		938
Gross unrealized losses, Within 1 year	(1)	(53)
Gross unrealized losses, 1 to 5 years	0	(113)
Gross unrealized losses, 5 to 10 years	(2,069)	(2,417)
Gross unrealized losses, Over 10 years		0
Fair Value, Available-for-sale securities
Within 1 year	28,111	278,366
1 to 5 years	65,208	64,296
5 to 10 years	$ 117,890	67,538
Over 10 years		$ 50,938
Yield, Available-for-sale securities
Within 1 year	2.07%	1.33%
1 to 5 years	2.85%	2.47%
5 to 10 years	2.56%	1.96%
Over 10 years		3.00%
Municipal bonds due
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 2,337	$ 2,315
5 to 10 years	1,359	1,335
Over 10 years	20,348	20,363
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	14	2
Gross unrealized gains, 5 to 10 years	60	38
Gross unrealized gains, Over 10 years	2,618	3,617
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	0
Fair Value, Available-for-sale securities
1 to 5 years	2,351	2,317
5 to 10 years	1,419	1,373
Over 10 years	$ 22,966	$ 23,980
Yield, Available-for-sale securities
1 to 5 years	1.23%	1.23%
5 to 10 years	2.05%	2.05%
Over 10 years	6.45%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 795,786	$ 978,955
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	8,553	17,118
Gross unrealized losses, Without single maturity date	(2,946)	(3,032)
Fair Value, Available-for-sale securities
Without single maturity date	$ 801,393	$ 993,041
Yield, Available-for-sale securities
Without single maturity date	2.87%	2.58%
Amortized Cost, Held-to-maturity securities
Without single maturity date	$ 1,651,528	$ 1,417,599
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	5,088	24,171
Gross unrealized losses, Without single maturity date	(25,871)	(214)
Fair Value, Held-to-maturity securities
Without single maturity date	$ 1,630,745	$ 1,441,556
Yield, Held-to-maturity securities
Without a single maturity date	3.17%	3.18%
Commercial MBS
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 8,350	$ 80,318
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	0	0
Gross unrealized losses, Without single maturity date	(41)	(448)
Fair Value, Available-for-sale securities
Without single maturity date	$ 8,309	$ 79,870
Yield, Available-for-sale securities
Without single maturity date	3.23%	1.91%