Share Based Compensation - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)	0.6	0.8
Shares granted (in shares)	0.2	0.2
Period of payment of award	3 years
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment of award, percentage	0.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Payment of award, percentage	200.00%
Target payout percentage	100.00%
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost	$ 10.4
Weighted-average period of total unrecognized compensation cost, in years	1 year 4 months 24 days
Unvested Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation cost	13.4
Weighted-average period of total unrecognized compensation cost, in years	1 year 1 month 6 days
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares)	0.1
Total unrecognized compensation cost	$ 1.9
Weighted-average period of total unrecognized compensation cost, in years	1 year 2 months 12 days