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Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jul. 31, 2012	Jul. 31, 2011	Jul. 31, 2012	Jul. 31, 2011
Income Taxes [Line Items]
Effective tax rate	30.00%	27.00%	30.00%	26.80%
Increase (decrease) in provision for income taxes, percentage	(14.50%)		6.90%
Provision for income taxes	$ 16,370	$ 19,142	$ 39,324	$ 36,798
Federal statutory rate	35.00%

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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- Definition

The domestic federal statutory tax rate applicable under enacted tax laws to the Company's pretax income from continuing operations for the period. The "statutory" tax rate is the regular tax rate if there are alternative tax systems.

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- Definition

The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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