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SCHEDULE II - Condensed Financial Information Of Registrant Condensed Balance Sheets (Parent Company) (Parenthetical) (Detail) - $ / shares
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]        
Common Shares, par value (USD per share) $ 1.00   $ 1 $ 1
Common Shares, authorized (shares) 900,000,000.0   900,000,000.0  
Common Shares, issued (shares) 797,500,000   797,500,000  
Common Shares, treasury shares (shares) 214,300,000   215,800,000  
Parent Company        
Condensed Financial Statements, Captions [Line Items]        
Common Shares, par value (USD per share) $ 1.00   $ 1.00  
Common Shares, authorized (shares) 900,000,000.0   900,000,000.0  
Common Shares, issued (shares) 797,500,000   797,500,000  
Common Shares, treasury shares (shares) 214,300,000   215,800,000  
Series B Preferred Stock        
Condensed Financial Statements, Captions [Line Items]        
Preferred Stock, Shares Issued   500,000    
Preferred Stock, Liquidation Preference Per Share   $ 1,000 $ 0  
Serial Preferred Stock        
Condensed Financial Statements, Captions [Line Items]        
Preferred Stock, Shares Authorized 20,000,000   20,000,000  
Serial Preferred Stock | Parent Company        
Condensed Financial Statements, Captions [Line Items]        
Preferred Stock, Shares Authorized 20,000,000   20,000,000  
Serial Preferred Stock | Series B Preferred Stock        
Condensed Financial Statements, Captions [Line Items]        
Preferred Stock, No Par Value $ 0   $ 0 $ 0
Preferred Stock, Shares Authorized 500,000.0   0  
Preferred Stock, Shares Issued 500,000.0   0  
Preferred Stock, Shares Outstanding 500,000.0   0  
Preferred Stock, Liquidation Preference Per Share $ 1,000   $ 0  
Serial Preferred Stock | Series B Preferred Stock | Parent Company        
Condensed Financial Statements, Captions [Line Items]        
Preferred Stock, No Par Value $ 0   $ 0  
Preferred Stock, Shares Authorized 500,000.0   0  
Preferred Stock, Shares Issued 500,000.0   0  
Preferred Stock, Shares Outstanding 500,000.0   0  
Preferred Stock, Liquidation Preference Per Share $ 1,000   $ 0