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SCHEDULE II - Condensed Financial Information Of Registrant Condensed Balance Sheets (Parent Company) (Parenthetical) (Detail) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
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) 215,800,000 217,600,000
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) 215,800,000 217,600,000