Total Gross Unrecognized Estimated Compensation Expense and Expected Weighted-Average Periods (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	$ 24.5
Stock Options
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	5.5
Weighted-average period for expense recognition	1 year 9 months 18 days
Restricted Stock
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	9.4
Weighted-average period for expense recognition	2 years 1 month 6 days
Performance Shares
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized estimated compensation expense, gross amount	$ 9.6
Weighted-average period for expense recognition	1 year 9 months 18 days