Share-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 3.0	$ 2.0	$ 9.1	$ 6.9
Tax benefits	$ 1.1	$ 0.8	$ 3.4	$ 2.6
Vesting period for the 2012 SAR grant			3 years
Portion of an award vesting for the 2012 SAR grant			1/3 vesting after each year
Stock appreciation rights (SARs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, Grants in period			350,200
Restricted stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, Grants in period			63,380
Performance shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, Grants in period			76,780
Phantom stock units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, equity instruments other than options, Grants in period			15,420