Risk Management and Derivatives - Additional Information (Detail) (USD $)	9 Months Ended
	Feb. 28, 2015	May 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Maximum term over which the Company is hedging exposures to the variability of cash flows for its forecasted and recorded transactions (in months)	27 months
Percentage of anticipated exposures hedged (percent)	100.00%
Deferred net gains (net of tax) on both outstanding and matured derivatives accumulated in other comprehensive income are expected to be reclassified to net income during the next twelve months as a result of underlying hedged transactions also being recorded in net income	$ 577,000,000
Aggregate fair value of derivative instruments in net liability position	2,000,000
Collateral received from counterparties to hedging instruments	769,000,000	0
Embedded derivatives
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total notional amount of outstanding derivatives	151,000,000
Undesignated Derivative Instruments
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total notional amount of outstanding derivatives	5,000,000,000
Minimum
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Typical time period that anticipated exposures are hedged against (in months)	12 months
Minimum fair value of outstanding derivative above which the credit related contingent features require the derivative party to post collateral	50,000,000
Maximum
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Typical time period that anticipated exposures are hedged against (in months)	24 months
Fair Value Hedging [Member] | Interest rate swap
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total notional amount of outstanding derivatives	100,000,000
Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total notional amount of outstanding derivatives	10,700,000,000
Cash Flow Hedging | Designated as Hedging Instrument | Interest rate swap
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Total notional amount of outstanding derivatives	750,000,000
Securities Pledged as Collateral
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Collateral received from counterparties to hedging instruments	77,000,000
Accrued Liabilities
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Collateral received from counterparties to hedging instruments	769,000,000
Accrued Liabilities | Interest rate swap
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Collateral received from counterparties to hedging instruments	33,000,000
Cash and Cash Equivalents
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Collateral received from counterparties to hedging instruments	769,000,000
Cash and Cash Equivalents | Interest rate swap
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Collateral received from counterparties to hedging instruments	$ 33,000,000