Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Jul. 31, 2012	Jul. 31, 2012
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased and retired (in shares)	3.4	5.9
Common Stock shares remained available for repurchase under repurchase plans (in shares)	38.8	38.8
Stock Repurchase Program, Number of Shares Authorized to be Repurchased	30.0	30.0
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchased shares of its common stock on the open market, value	(94.5)	(148.6)
Retained Earnings [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchased shares of its common stock on the open market, value	(16.6)	(61.7)
Common Stock Repurchase Program [member]
Equity, Class of Treasury Stock [Line Items]
Repurchased shares of its common stock on the open market, average repurchase price per share (in usd per share)	32.23	35.41