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Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Jan. 31, 2012
Jul. 31, 2011
Available-for-sale debt securities:    
Total available-for-sale debt securities $ 1,348 $ 1,898
Municipal bonds [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 391 434
Municipal auction rate securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 54 59
Corporate notes [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 342 288
U.S. agency securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 124 152
Fair Value, Measurements, Recurring [Member]
   
Assets:    
Cash equivalents 283 854
Available-for-sale debt securities:    
Total available-for-sale debt securities 911 933
Total assets measured at fair value on a recurring basis 1,194 1,787
Liabilities:    
Senior notes 1,077 [1] 1,084 [1]
Fair Value, Measurements, Recurring [Member] | Municipal bonds [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 391 434
Fair Value, Measurements, Recurring [Member] | Municipal auction rate securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 54 59
Fair Value, Measurements, Recurring [Member] | Corporate notes [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 342 288
Fair Value, Measurements, Recurring [Member] | U.S. agency securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 124 152
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
   
Assets:    
Cash equivalents 283 854
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Total assets measured at fair value on a recurring basis 283 854
Liabilities:    
Senior notes 0 [1] 0 [1]
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Municipal bonds [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Municipal auction rate securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate notes [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. agency securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
   
Assets:    
Cash equivalents 0 0
Available-for-sale debt securities:    
Total available-for-sale debt securities 857 874
Total assets measured at fair value on a recurring basis 857 874
Liabilities:    
Senior notes 1,077 [1] 1,084 [1]
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal bonds [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 391 434
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal auction rate securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate notes [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 342 288
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. agency securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 124 152
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
   
Assets:    
Cash equivalents 0 0
Available-for-sale debt securities:    
Total available-for-sale debt securities 54 59
Total assets measured at fair value on a recurring basis 54 59
Liabilities:    
Senior notes 0 [1] 0 [1]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal bonds [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal auction rate securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 54 59
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate notes [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities 0 0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. agency securities [Member]
   
Available-for-sale debt securities:    
Total available-for-sale debt securities $ 0 $ 0
[1] Carrying value on our balance sheets at January 31, 2012 and July 31, 2011 was $999 million. See Note 5.