Total Gross Unrecognized Estimated Compensation Expense and Expected Weighted-Average Periods (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 Year
Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	$ 4.4
Weighted-average period for expense recognition	1.8
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	6.5
Weighted-average period for expense recognition	1.6
Performance shares
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	$ 5.8
Weighted-average period for expense recognition	3.2