Stock Based Compensation Plans - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011 Y	Dec. 31, 2010 Y
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance under the Incentive Plan, shares	12,250,000
Period of expiration of stock options granted to employees and non-employee directors, in years, maximum	10
Period of expiration of stock options granted to employees, in years	7	7	7
Period of expiration of stock options granted to non-employee directors, in years	5	5	5
Total fair value of restricted share awards vested during the period	$ 4.0	$ 4.2	$ 1.9
Income tax benefits realized	4.2	5.1	6.3
Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average estimated grant-date fair values of employee and director stock options granted (per share)	$ 25.24	$ 28.10	$ 16.64
Total pre-tax intrinsic value of stock options exercised	7.5	12.2	16.6
Pre-tax unrecognized compensation expense, net of estimated forfeitures	3.6
Weighted-average period of pre-tax unrecognized compensation expense for stock options to be recognized as expense, years	1 year 10 months 24 days
Restricted Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pre-tax unrecognized compensation expense, net of estimated forfeitures	$ 3.3
Weighted-average period of pre-tax unrecognized compensation expense for stock options to be recognized as expense, years	1 year 8 months 12 days
Period restricted share awards generally cliff vest after date of grant, years	3