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Significant accounting policies and practices - Narrative (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Jan. 01, 2018
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Significant Accounting Policies [Line Items]          
Unpaid losses and loss adjustment expenses $ 61,122   $ 53,379 $ 50,519 $ 48,208
Income tax liabilities 56,607   77,442    
Retained earnings 255,786   210,846    
Cash and cash equivalents 31,583   28,048 67,161 60,033
Short-term investments in U.S. Treasury Bills $ 84,400        
Percentage of LIFO inventory 41.00%        
Assets $ 702,095   620,854 $ 552,257  
Liabilities 350,141   335,426    
Discontinuation of Discounting Workers' Compensation Claims Liabilities Assumed Under Reinsurance Contracts [Member] | Restatement Adjustment [Member]          
Significant Accounting Policies [Line Items]          
Unpaid losses and loss adjustment expenses         1,430
Income tax liabilities         (502)
Retained earnings         $ (931)
U.S. Treasury Bills [Member]          
Significant Accounting Policies [Line Items]          
Investments 90,100        
Cash and cash equivalents $ 5,700        
Minimum [Member]          
Significant Accounting Policies [Line Items]          
Insurance interest rate assumption 1.00%        
Maximum [Member]          
Significant Accounting Policies [Line Items]          
Insurance interest rate assumption 7.00%        
Track structure and other roadway [Member] | Railroad [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 9 years        
Track structure and other roadway [Member] | Railroad [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 100 years        
Locomotives, freight cars and other equipment [Member] | Railroad [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 6 years        
Locomotives, freight cars and other equipment [Member] | Railroad [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 41 years        
Utility generation, transmission and distribution systems [Member] | Utilities and Energy [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 5 years        
Utility generation, transmission and distribution systems [Member] | Utilities and Energy [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 80 years        
Interstate natural gas pipeline assets [Member] | Utilities and Energy [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 3 years        
Interstate natural gas pipeline assets [Member] | Utilities and Energy [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 80 years        
Independent power plants and other assets [Member] | Utilities and Energy [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 3 years        
Independent power plants and other assets [Member] | Utilities and Energy [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 30 years        
Buildings and improvements [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 5 years        
Buildings and improvements [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 50 years        
Machinery and equipment [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 3 years        
Machinery and equipment [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 25 years        
Furniture, fixtures and other [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 3 years        
Furniture, fixtures and other [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 15 years        
Assets held for lease [Member] | Minimum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 6 years        
Assets held for lease [Member] | Maximum [Member]          
Significant Accounting Policies [Line Items]          
Estimated useful life (in years) 35 years        
Other Assets [Member]          
Significant Accounting Policies [Line Items]          
Unamortized balances of deferred policy acquisition costs $ 2,529   $ 1,991    
ASU 2016-01 [Member] | Subsequent Event [Member] | Equity Securities [Member]          
Significant Accounting Policies [Line Items]          
Retained earnings   $ 61,500      
Accumulated other comprehensive income, net unrealized appreciation of investments   (61,500)      
ASU 2014-09 [Member] | Subsequent Event [Member]          
Significant Accounting Policies [Line Items]          
Liabilities   3,500      
ASU 2014-09 [Member] | Subsequent Event [Member] | Primarily Property, Plant and Equipment [Member]          
Significant Accounting Policies [Line Items]          
Assets   $ 3,500