Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 2,085,416	$ 2,365,450
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	24,587	28,158
Gross unrealized loss	(12,917)	(13,045)
Fair Value, Available-for-sale securities
Available-for-sale securities, at fair value	$ 2,097,086	$ 2,380,563
Yield, Available-for-sale securities
Yield	2.13%	1.97%
Yield, Held-to-maturity securities
Investments	$ 3,643,503	$ 4,008,666
Gross unrealized gains on investments	36,945	38,674
Gross unrealized losses on investments	(19,013)	(29,357)
Total financial assets	$ 3,661,435	$ 4,017,983
Yield on investments	2.58%	2.46%
Obligations of U.S. government
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 44,763	$ 105,065
5 to 10 years	50,399	119,071
Over 10 years	173,258	262,832
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	1,467	1,923
Gross unrealized gains, 5 to 10 years	0	35
Gross unrealized gains, Over 10 years	0	0
Gross unrealized losses, 1 to 5 years	(257)	(274)
Gross unrealized losses, 5 to 10 years	(1,726)	(1,247)
Gross unrealized losses, Over 10 years	(3,361)	(4,941)
Fair Value, Available-for-sale securities
1 to 5 years	45,973	106,714
5 to 10 years	48,673	117,859
Over 10 years	$ 169,897	$ 257,891
Yield, Available-for-sale securities
1 to 5 years	2.72%	1.74%
5 to 10 years	1.04%	1.54%
Over 10 years	1.06%	1.23%
Equity securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 500	$ 500
1 to 5 years	99,922	99,922
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	19	17
Gross unrealized gains, 1 to 5 years	1,150	1,513
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	519	517
1 to 5 years	$ 101,072	$ 101,435
Yield, Available-for-sale securities
Within 1 year	1.80%	1.80%
1 to 5 years	1.90%	1.90%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Within 1 year	$ 274,957	$ 24,787
1 to 5 years	91,505	311,435
5 to 10 years	89,953	100,000
Over 10 years	50,000	69,950
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Within 1 year	258	191
Gross unrealized gains, 1 to 5 years	355	1,190
Gross unrealized gains, 5 to 10 years	1,167	876
Gross unrealized gains, Over 10 years	974	953
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 1 to 5 years	(113)	(58)
Gross unrealized losses, 5 to 10 years	(2,787)	(3,524)
Gross unrealized losses, Over 10 years	0	0
Fair Value, Available-for-sale securities
Within 1 year	275,215	24,978
1 to 5 years	91,747	312,567
5 to 10 years	88,333	97,352
Over 10 years	$ 50,974	$ 70,903
Yield, Available-for-sale securities
Within 1 year	1.02%	0.53%
1 to 5 years	1.85%	0.88%
5 to 10 years	2.00%	1.47%
Over 10 years	3.00%	3.00%
Municipal bonds due
Amortized Cost, Available-for-sale securities
1 to 5 years	$ 2,300	$ 2,285
5 to 10 years	1,319	1,303
Over 10 years	20,372	20,382
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	8	8
Gross unrealized gains, 5 to 10 years	34	7
Gross unrealized gains, Over 10 years	3,302	3,138
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	2,308	2,293
5 to 10 years	1,353	1,310
Over 10 years	$ 23,674	$ 23,520
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	$ 1,093,626	$ 1,144,787
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	15,593	18,222
Gross unrealized losses, Without single maturity date	(4,202)	(2,491)
Fair Value, Available-for-sale securities
Without single maturity date	$ 1,105,017	$ 1,160,518
Yield, Available-for-sale securities
Without single maturity date	2.58%	2.48%
Amortized Cost, Held-to-maturity securities
Without single maturity date	$ 1,558,087	$ 1,643,216
Gross Unrealized Gains / Losses, Held-to-maturity securities
Gross unrealized gains, Without single maturity date	12,358	10,516
Gross unrealized losses, Without single maturity date	(6,096)	(16,312)
Fair Value, Held-to-maturity securities
Without single maturity date	$ 1,564,349	$ 1,637,420
Yield, Held-to-maturity securities
Without a single maturity date	3.19%	3.19%
Other Commercial MBS
Amortized Cost, Available-for-sale securities
Without single maturity date	$ 92,542	$ 103,131
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, Without single maturity date	260	85
Gross unrealized losses, Without single maturity date	(471)	(510)
Fair Value, Available-for-sale securities
Without single maturity date	$ 92,331	$ 102,706
Yield, Available-for-sale securities
Without single maturity date	1.73%	1.51%