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Earnings Per Share - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Senior convertible notes due 2040 [Member]
        
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Senior convertible notes convertible to common shares price $ 15.55   $ 15.55  
Conversion price per share greater than applicable trading day     125.00%  
Stock options [Member]
        
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Options to purchase common shares not included in computation of diluted EPS 2.7 2.8 2.7 2.8
Forward equity [Member]
        
Dilutive Securities Included And Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Forward equity agreement entered, not included in the computation of diluted EPS 39.1 19.0 39.1 19.0