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Common Stock - Schedule of Additional Information on Compensation Costs and Income Tax Benefits and Excess Tax Benefits Related to Stock-Based Compensation Awards (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Compensation costs	$ 12.0	$ 9.1	$ 7.4
Income tax benefits	4.6	3.5	2.9
Excess tax benefits	$ 2.6	$ 1.7	$ 0.8

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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