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Earnings Per Share - Additional Information (Detail) - $ / shares
shares in Millions
1 Months Ended 12 Months Ended
May. 31, 2013
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Senior Convertible Notes due 2040 [Member]        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Senior convertible notes convertible to common shares price     $ 14.85  
Restricted Stock Awards [Member]        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Securities excluded from basic average shares outstanding   0.7 1.2 1.8
Stock options [Member]        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Securities excluded from basic average shares outstanding   2.8 3.0 2.7
Forward Equity Agreements [Member]        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Securities excluded from basic average shares outstanding 39.1