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Financial Instruments - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Oct. 26, 2018
Oct. 27, 2017
Oct. 26, 2018
Oct. 27, 2017
Apr. 27, 2018
Apr. 28, 2017
Investments [Abstract]            
Credit loss portion of other-than-temporary impairments $ 0   $ 0   $ 0  
Reductions for available-for-sale securities sold 0 $ 0 0 $ 0    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cumulative effect of change in accounting principle           $ 296,000,000
Equity Investments            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Net gains (losses) recognized (4,000,000)   94,000,000      
Net realized gains (losses) on equity and other investments sold during the period 0   45,000,000      
Net unrealized gains (losses) on equity and other investments still held $ (4,000,000)   $ 49,000,000      
Accumulated Other Comprehensive Loss            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cumulative effect of change in accounting principle         47,000,000  
Accumulated Other Comprehensive Loss | Accounting Standards Update 2016-01            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cumulative effect of change in accounting principle         47,000,000  
Retained Earnings | Accounting Standards Update 2016-01            
New Accounting Pronouncements or Change in Accounting Principle [Line Items]            
Cumulative effect of change in accounting principle         $ (47,000,000)