Fair Value Measurements (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Jul. 31, 2016	Jan. 31, 2016
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative agreements - assets	$ 0	$ 0
Derivative agreements - liabilities	0	0
Total	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative agreements - assets	253	213
Derivative agreements - liabilities	0	(82)
Total	253	131
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative agreements - assets	0	0
Derivative agreements - liabilities	0	0
Total	$ 0	$ 0