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Significant accounting policies - Narrative (Detail) - USD ($)
$ in Millions
12 Months Ended 156 Months Ended
Dec. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Significant Accounting Policies [Line Items]      
Percentage of LIFO inventory 49.00% 49.00%  
Percentage of FIFO inventory 29.00% 29.00%  
Difference between LIFO cost and FIFO cost for inventory measured using LIFO $ 715 $ 715 $ 857
Workers compensation discount rate, before January 1, 2003 4.50% 4.50%  
Workers compensation discount rate, since January 1, 2003   1.00%  
Life insurance interest rate assumption, lower end of range 1.00%    
Life insurance interest rate assumption, higher end of range 7.00%    
Annuities interest rate assumption, lower end of range 1.00%    
Annuities interest rate assumption, higher end of range 7.00%    
Other Assets [Member]      
Significant Accounting Policies [Line Items]      
Unamortized balances of deferred policy acquisition costs $ 1,920 $ 1,920 $ 1,722