Available-for-Sale Securities - Interest-bearing Investments in Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 385,598	$ 779,809
Gross Unrealized Gains	176	640
Gross Unrealized Losses	577	67
Fair Value	385,197	780,382
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	98,367	95,852
Gross Unrealized Gains	176	339
Gross Unrealized Losses	158	37
Fair Value	98,385	96,154
U.S. Government Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,558	105,377
Gross Unrealized Gains		151
Gross Unrealized Losses	195	15
Fair Value	84,363	105,513
U.S. Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	60,794	48,339
Gross Unrealized Gains		69
Gross Unrealized Losses	148	15
Fair Value	60,646	48,393
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,504	449,288
Fair Value	52,504	449,288
Asset Backed Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,108	30,121
Gross Unrealized Gains		73
Gross Unrealized Losses	34
Fair Value	47,074	30,194
Commercial Paper and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	33,448	42,498
Fair Value	33,448	42,498
Supranational and Sovereign Debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	8,819	8,334
Gross Unrealized Gains		8
Gross Unrealized Losses	42
Fair Value	$ 8,777	$ 8,342

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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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
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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