Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 801,359	$ 977,669
Gross Unrealized Gains	456	242
Gross Unrealized Losses	133	272
Fair Value	801,682	977,639
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	470,286	649,745
Fair Value	470,286	649,745
U.S. Government Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	117,235	118,652
Gross Unrealized Gains	217	88
Gross Unrealized Losses		32
Fair Value	117,452	118,708
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101,613	102,387
Gross Unrealized Gains	123	152
Gross Unrealized Losses	133	119
Fair Value	101,603	102,420
U.S. Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	45,105	41,658
Gross Unrealized Gains	76
Gross Unrealized Losses		9
Fair Value	45,181	41,649
Asset Backed Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,195	29,160
Gross Unrealized Gains	20
Gross Unrealized Losses		65
Fair Value	29,215	29,095
Commercial Paper and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	27,502	28,509
Fair Value	27,502	28,509
Supranational and Sovereign Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,423	5,804
Gross Unrealized Gains	20
Gross Unrealized Losses		5
Fair Value	$ 10,443	5,799
Government-Guaranteed Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,185
Gross Unrealized Gains		2
Fair Value		1,187
Mortgages [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		569
Gross Unrealized Losses		42
Fair Value		$ 527

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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