Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Apr. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Tax Cuts And Jobs Act Of 2017, Incomplete Accounting, Provisional Income Tax Expense Benefit	$ 100,000,000	$ 207,000,000
Adoption of new accounting standards on February 1, 2018	924,000,000
Restricted cash	300,000,000	$ 300,000,000	$ 300,000,000
Accounting Standards Update 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting standards on February 1, 2018	$ 2,600,000,000