Share-Based Payment Arrangements - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance under equity compensation plans	4,200,000
Recognized stock-based compensation	$ 11.0	$ 7.2	$ 4.1
Recognized tax benefit related to the share-based compensation	3.9	2.5	1.4
Percentage where a option price is equal to market price	110.00%
Term of option award vest, contractual term	10 years
Term of option award vest for continuous service	5 years
Options expiration date in years	10 years
Options expiration date in months	6 months
Intrinsic value of options exercised	0.8	0.3	0.8
Weighted-average grant-date fair values of options granted	$ 8.43	$ 8.64	$ 10.73
Unrecognized compensation cost	37.2
Weighted-average period in years	3 years 8 months 12 days
Total fair value of shares vested	$ 2.3	$ 1.3
Shareholder return period	3 years
Performance Share Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted to key members of executives and senior management	27,797
Fair Value of shares granted	$ 36.16
Percentage of maximum number of shares earned of target award	200.00%
2005 Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares can be granted under long term incentive Plan	5,000,000
Share-based compensation arrangement by share-based payment award, maximum annual grant available	2.00%