Summary of Significant Accounting Policies - Changes in accounting policies (Details) $ in Millions	12 Months Ended
	Jan. 31, 2015 USD ($)
Other Current Assets [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 703
Other Current Assets [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(2)
Other Current Assets [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(252)
Other Current Assets [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	449
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	684
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	681
Other Assets [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	583
Other Assets [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(6)
Other Assets [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	252
Other Assets [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	829
Assets, Total [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	15,256
Assets, Total [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(8)
Assets, Total [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Assets, Total [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	15,245
Long-term Debt [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	1,580
Long-term Debt [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(8)
Long-term Debt [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Long-term Debt [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	1,572
Long-Term Liabilities Held-for-Sale [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	18
Long-Term Liabilities Held-for-Sale [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0
Long-Term Liabilities Held-for-Sale [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Long-Term Liabilities Held-for-Sale [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	15
Liabilities, Total [Member] | As reported [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	15,256
Liabilities, Total [Member] | Accounting Standards Update 2015-03 and 2015-15 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(8)
Liabilities, Total [Member] | Accounting Standards Update 2015-17 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(3)
Liabilities, Total [Member] | Adjusted Balance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ 15,245