Stock Option Plans - Additional information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011 Y	Mar. 08, 2011	Dec. 31, 2010 Y	Mar. 02, 2010	Mar. 16, 2012 Officer and Key Employees [Member]	Dec. 31, 2012 Incentive Plan Prior to 2007 [Member]	Dec. 31, 2012 Non Qualified Plan Prior to 2007 [Member] Officer and Key Employees [Member]	Dec. 31, 2012 Prior to 2009 Options [Member] Y	Dec. 31, 2012 Black-Scholes Option Pricing Model [Member]	Dec. 31, 2011 Black-Scholes Option Pricing Model [Member]	Dec. 31, 2010 Black-Scholes Option Pricing Model [Member]	Dec. 31, 2012 Long Term Incentive Plan [Member]	Dec. 31, 2012 Performance-Based Restricted Stock [Member]
Stock Option Plans [Line Items]
Maximum number of shares available	352,000	224,000		216,000									900,000
Maximum number of shares granted during any fiscal year to any person	200,000													100,000
Maximum amount payable for the shares granted during any fiscal year to any person														$ 3.0
Maximum period for the exercise of stock options, years	7
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant	100.00%
Shares available for grant			851,000		858,000	1,355,000							10,000,000
Total intrinsic value of options exercised	26.0	10.8		7.8
Total unrecognized compensation cost related to nonvested options	17.1
Weighted average period, years	4 years
Stock options granted, exercise rate per year			20.00%		20.00%		10.00%	10.00%
Stock options granted, exercise percentage, year one						34.00%
Stock options granted, exercise percentage, year two						33.00%
Stock options granted, exercise percentage, year three						33.00%
Number of years options expire, maximum	7	7		7					10
Accelerated vesting in the event of death, disability or retirement								100.00%
Compensation expense related to stock option grants	$ 7.2	$ 7.1		$ 7.7
Weighted average fair value per option for all options										$ 5.49	$ 5.25	$ 3.90