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Marketable Securities - Additional Information (Detail) (USD $)	3 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Jan. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale and maturities of available-for-sale securities	$ 155,801,000	$ 53,829,000
Amortization of discounts and premiums, net	2,192,000	1,235,000
Estimated fair value of available-for-sale securities	364,709,000	208,287,000	378,071,000
Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available-for-sale securities	149,771,000	91,231,000	149,585,000
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale securities	4,600,000
Estimated fair value of available-for-sale securities		4,330,000	4,330,000
Auction Rate Securities | Long Term Investments
Schedule of Available-for-sale Securities [Line Items]
Par value of available-for-sale securities		4,900,000	4,900,000
Estimated fair value of available-for-sale securities		4,330,000	4,330,000
Other Expense Income Net
Schedule of Available-for-sale Securities [Line Items]
Net realized gain (loss)	$ 215,000	$ 127,000

X

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Amount of noncash expense included in interest expense to amortize debt discount and premium associated with the related debt instruments. Excludes amortization of financing costs. Alternate captions include noncash interest expense.

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