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Fair Value - Additional Information (Detail) (USD $)				3 Months Ended					3 Months Ended
	Jul. 31, 2013	Jan. 31, 2013	Jul. 31, 2012	Apr. 30, 2013 Auction Rate Securities	Jan. 31, 2013 Auction Rate Securities	Jul. 31, 2012 Auction Rate Securities	Jan. 31, 2013 Fair Value, Inputs, Level 3	Jul. 31, 2012 Fair Value, Inputs, Level 3	Jul. 31, 2013 Fair Value, Inputs, Level 3 Auction Rate Securities	Jan. 31, 2013 Fair Value, Inputs, Level 3 Auction Rate Securities	Jul. 31, 2012 Fair Value, Inputs, Level 3 Auction Rate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from sale of available-for-sale securities				$ 4,600,000					$ 4,600,000
Amortized cost of available-for-sale securities	442,558,000	378,393,000	227,773,000							4,925,000,000	4,925,000,000
Estimated fair value of available-for-sale securities	$ 442,565,000	$ 378,071,000	$ 227,554,000		$ 4,330,000	$ 4,330,000	$ 4,330,000	$ 4,330,000		$ 4,330,000	$ 4,330,000

X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558



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-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

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-Publisher FASB

-Name Accounting Standards Codification

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-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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X

- Definition

This item represents the cost of debt and equity 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 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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X

- Definition

The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133



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