Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 779,809	$ 801,359
Gross Unrealized Gains	640	456
Gross Unrealized Losses	67	133
Fair Value	780,382	801,682
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	449,288	470,286
Fair Value	449,288	470,286
U.S. Government Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	105,377	117,235
Gross Unrealized Gains	151	217
Gross Unrealized Losses	15
Fair Value	105,513	117,452
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	95,852	101,613
Gross Unrealized Gains	339	123
Gross Unrealized Losses	37	133
Fair Value	96,154	101,603
U.S. Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,339	45,105
Gross Unrealized Gains	69	76
Gross Unrealized Losses	15
Fair Value	48,393	45,181
Asset Backed Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,121	29,195
Gross Unrealized Gains	73	20
Fair Value	30,194	29,215
Commercial Paper and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	42,498	27,502
Fair Value	42,498	27,502
Supranational and Sovereign Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,334	10,423
Gross Unrealized Gains	8	20
Fair Value	$ 8,342	$ 10,443

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
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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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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