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Investment gains/losses - Narrative (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Gain (Loss) on Investments [Line Items]        
Investment gains/losses $ 362 $ 2,366 $ 459 $ 3,497
Equity Securities [Member] | PSX and GHC [Member] | PSPI and WPLG Inc. [Member]        
Gain (Loss) on Investments [Line Items]        
Investment gains/losses       $ 2,100
Equity Securities [Member] | Graham Holding Company [Member] | WPLG Inc. [Member]        
Gain (Loss) on Investments [Line Items]        
Investment gains/losses   $ 1,100