Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation - Additional Information (Detail)

v2.4.0.8
Share-Based Compensation - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]        
Share-based compensation expense $ 2.0 $ 0.6 $ 4.9 $ 1.8
Unrecognized compensation cost related to nonvested common shares $ 16.9   $ 16.9  
Weighted average period for recognition of cost     2 years 4 months 24 days