Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,444,338	$ 1,909,184
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	11,815	25,422
Gross unrealized loss	(14,218)	(11,712)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 1,441,935	$ 1,922,894
Yield, Available-for-sale securities
Yield	2.49%	2.22%
Yield, Held-to-maturity securities
Investments	$ 3,196,348	$ 3,326,783
Gross unrealized gains on investments	15,674	49,593
Gross unrealized losses on investments	(46,995)	(11,926)
Total financial assets	$ 3,165,027	$ 3,364,450
Yield on investments	2.84%	2.62%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 23,102	$ 21,284
1 to 5 years	35,788	12,477
5 to 10 years	42,301	48,134
Over 10 years	149,673	182,051
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	428	0
Gross unrealized gains, 1 to 5 years	354	1,027
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, Over 10 years	0	27
Gross unrealized losses, Within 1 year	(27)	(59)
Gross unrealized losses, 1 to 5 years	(1,151)	(11)
Gross unrealized losses, 5 to 10 years	(955)	(1,589)
Gross unrealized losses, Over 10 years	(2,245)	(3,990)
Fair Value, Available-for-sale securities
Within 1 year	23,503	21,225
1 to 5 years	34,991	13,493
5 to 10 years	41,346	46,545
Over 10 years	$ 147,428	$ 178,088
Yield, Available-for-sale securities
Within 1 year	3.55%	0.81%
1 to 5 years	2.15%	7.94%
5 to 10 years	1.75%	1.14%
Over 10 years	1.44%	1.33%
Equity securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 0	$ 0
1 to 5 years	500	100,422
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	0	0
Gross unrealized gains, 1 to 5 years	22	1,402
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 1 to 5 years	0	0
Fair Value, Available-for-sale securities
Within 1 year	0	0
1 to 5 years	$ 522	$ 101,824
Yield, Available-for-sale securities
Within 1 year	0.00%	0.00%
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	63,516	63,481
5 to 10 years	69,957	69,955
Over 10 years	50,000	50,000
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	52	325
Gross unrealized gains, 1 to 5 years	951	928
Gross unrealized gains, 5 to 10 years	0	0
Gross unrealized gains, Over 10 years	0	938
Gross unrealized losses, Within 1 year	0	(53)
Gross unrealized losses, 1 to 5 years	(50)	(113)
Gross unrealized losses, 5 to 10 years	(3,361)	(2,417)
Gross unrealized losses, Over 10 years	(1,134)	0
Fair Value, Available-for-sale securities
Within 1 year	28,146	278,366
1 to 5 years	64,417	64,296
5 to 10 years	66,596	67,538
Over 10 years	$ 48,866	$ 50,938
Yield, Available-for-sale securities
Within 1 year	1.84%	1.33%
1 to 5 years	2.57%	2.47%
5 to 10 years	2.06%	1.96%
Over 10 years	3.00%	3.00%
Municipal bonds due
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 2,322	$ 2,315
5 to 10 years	1,343	1,335
Over 10 years	20,358	20,363
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	0	2
Gross unrealized gains, 5 to 10 years	0	38
Gross unrealized gains, Over 10 years	2,462	3,617
Gross unrealized losses, 1 to 5 years	(8)	0
Gross unrealized losses, 5 to 10 years	(18)	0
Gross unrealized losses, Over 10 years	0	0
Fair Value, Available-for-sale securities
1 to 5 years	2,314	2,317
5 to 10 years	1,325	1,373
Over 10 years	$ 22,820	$ 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	$ 904,287	$ 978,955
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	7,546	17,118
Gross unrealized losses, Without single maturity date	(4,986)	(3,032)
Fair Value, Available-for-sale securities
Without single maturity date	$ 906,847	$ 993,041
Yield, Available-for-sale securities
Without single maturity date	2.65%	2.58%
Amortized Cost, Held-to-maturity securities
Without single maturity date	$ 1,752,010	$ 1,417,599
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	3,859	24,171
Gross unrealized losses, Without single maturity date	(32,777)	(214)
Fair Value, Held-to-maturity securities
Without single maturity date	$ 1,723,092	$ 1,441,556
Yield, Held-to-maturity securities
Without a single maturity date	3.18%	3.18%
Commercial MBS
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 53,097	$ 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	(283)	(448)
Fair Value, Available-for-sale securities
Without single maturity date	$ 52,814	$ 79,870
Yield, Available-for-sale securities
Without single maturity date	1.95%	1.91%