Common Stock and Treasury Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			58 Months Ended	1 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011	Feb. 22, 2012 Stock repurchase	Dec. 31, 2011 Restricted share awards (RSAs)	Dec. 31, 2010 Restricted share awards (RSAs)	Dec. 31, 2009 Restricted share awards (RSAs)
Stockholders Equity Note [Line Items]
Maximum stock authorized to purchase under stock repurchase program	$ 210.0				$ 262.1
Repurchase of common stock, shares				8,082,180
Repurchase of common stock, value				187.1
Maximum remaining dollar value of shares authorized for purchase under the stock repurchase program	22.9				75.0
Increase in maximum stock authorized to purchase under stock repurchase program					$ 52.1
Number shares issued pursuant to stock option exercises	361,093	235,986	150,134
Number shares issued pursuant to restricted share award grants						6,300	25,950	23,500