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Basis of Presentation - Additional Information (Detail) - shares
shares in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Subsidiary or Equity Method Investee [Line Items]        
Shares excluded from diluted EPS calculation 0.4 3.3 0.4 4.7
CNA Financial [Member]        
Subsidiary or Equity Method Investee [Line Items]        
Subsidiary ownership percentage     89.00%  
Diamond Offshore Drilling, Inc. [Member]        
Subsidiary or Equity Method Investee [Line Items]        
Subsidiary ownership percentage     53.00%  
Boardwalk Pipeline Partners, LP [Member]        
Subsidiary or Equity Method Investee [Line Items]        
Subsidiary ownership percentage     51.00%  
Consolidated Container Company [Member]        
Subsidiary or Equity Method Investee [Line Items]        
Subsidiary ownership percentage     99.00%