Derivative Financial Instruments (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
		Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015	May. 31, 2015
Derivative Financial Instruments
Notional amount		$ 10,230,641,000		$ 10,230,641,000		$ 9,625,533,000
Notional balance of derivative assets		3,499,000,000		3,499,000,000		3,448,615,000
Notional balance of derivative liabilities		6,731,641,000		6,731,641,000		6,176,918,000
Derivative assets		104,535,000		104,535,000		115,276,000
Derivative liabilities		(688,998,000)		(688,998,000)		(408,382,000)
Total fair value		(584,463,000)		(584,463,000)		(293,106,000)
Derivative forward value				(290,952,000)	$ (159,832,000)
Derivative gains (losses)		(243,036,000)	$ (98,770,000)	(356,237,000)	(223,209,000)
Notional amount		10,230,641,000		10,230,641,000		9,625,533,000
Interest rate swaps
Derivative Financial Instruments
Notional amount		10,230,641,000		$ 10,230,641,000		$ 9,625,533,000
Weighted-average rate paid (as a percent)				2.29%		2.21%
Weighted-average rate received (as a percent)				1.36%		1.40%
Derivative assets		104,535,000		$ 104,535,000
Derivative liabilities		(688,998,000)		(688,998,000)
Gross amounts of recognized assets/liabilities		104,535,000		104,535,000		$ 115,276,000
Gross amount of derivative asset offset		0		0		0
Net amount of derivative asset presented on the balance sheet		104,535,000		104,535,000		115,276,000
Gross amount of derivative asset's pledged securities not offset		104,535,000		104,535,000		115,276,000
Gross amount of derivative asset's pledged cash not offset		0		0		0
Net amount of derivative asset		0		0		0
Gross amounts of recognized assets/liabilities		688,998,000		688,998,000		408,382,000
Gross amount offset against derivative liability		0		0		0
Net amount of derivative liabilities presented on the balance sheet		688,998,000		688,998,000		408,382,000
Gross amount of derivative liability's right to reclaim securities not offset		104,535,000		104,535,000		115,276,000
Gross amount of derivative liability's right to reclaim cash not offset		0		0		0
Net amount of derivative liabilities		584,463,000		584,463,000		293,106,000
Derivative cash settlements		(22,556,000)	(21,512,000)	(65,285,000)	(63,377,000)
Derivative forward value		(220,480,000)	(77,258,000)	(290,952,000)	(159,832,000)
Derivative gains (losses)		(243,036,000)	$ (98,770,000)	(356,237,000)	$ (223,209,000)
Notional amount		10,230,641,000		10,230,641,000		9,625,533,000
Aggregate fair value of interest rate swaps with rating triggers that were in net liability position		412,000,000		412,000,000
Derivative, Net Asset Position, Aggregate Fair Value		0		0
Pay-fixed swaps
Derivative Financial Instruments
Notional amount		6,731,641,000		$ 6,731,641,000		$ 5,776,533,000
Weighted-average rate paid (as a percent)				2.95%		3.15%
Weighted-average rate received (as a percent)				0.61%		0.28%
Notional amount		6,731,641,000		$ 6,731,641,000		$ 5,776,533,000
Receive-fixed swaps
Derivative Financial Instruments
Notional amount		3,499,000,000		$ 3,499,000,000		$ 3,849,000,000
Weighted-average rate paid (as a percent)				1.01%		0.79%
Weighted-average rate received (as a percent)				2.82%		3.09%
Notional amount		3,499,000,000		$ 3,499,000,000		$ 3,849,000,000
Forward Contracts
Derivative Financial Instruments
Notional amount		40,000,000		40,000,000
Notional amount		40,000,000		40,000,000
Counterparty Group | Interest rate swaps
Derivative Financial Instruments
Notional amount		7,181,720,000		7,181,720,000
Notional amount		7,181,720,000		7,181,720,000
Entity's required payment		(404,213,000)		(404,213,000)
Entity's amount collected		0		0
Entity's net payment		(404,213,000)		(404,213,000)
Counterparty Group | Moody's, A3 Rating Standard Poor's A- rating | Interest rate swaps | Mutual rating trigger falls to
Derivative Financial Instruments
Notional amount	[1]	63,295,000		63,295,000
Notional amount	[1]	63,295,000		63,295,000
Entity's required payment	[1]	(19,794,000)		(19,794,000)
Entity's amount collected	[1]	0		0
Entity's net payment	[1]	(19,794,000)		(19,794,000)
Counterparty Group | Baa1 | Interest rate swaps | Mutual rating trigger falls to
Derivative Financial Instruments
Notional amount		6,556,100,000		6,556,100,000
Notional amount		6,556,100,000		6,556,100,000
Entity's required payment		(348,858,000)		(348,858,000)
Entity's amount collected		0		0
Entity's net payment		(348,858,000)		(348,858,000)
Counterparty Group | Moody's Baa 2 Rating Standard Poor's BBB Rating | Interest rate swaps | Mutual rating trigger falls to
Derivative Financial Instruments
Notional amount	[2]	162,325,000		162,325,000
Notional amount	[2]	162,325,000		162,325,000
Entity's required payment	[2]	(3,955,000)		(3,955,000)
Entity's amount collected	[2]	0		0
Entity's net payment	[2]	(3,955,000)		(3,955,000)
Counterparty Group | Moodys Baa3 Rating Standard Poor's BB Rating | Interest rate swaps | Mutual rating trigger falls to
Derivative Financial Instruments
Notional amount		400,000,000		400,000,000
Notional amount		400,000,000		400,000,000
Entity's required payment		(31,606,000)		(31,606,000)
Entity's amount collected		0		0
Entity's net payment		$ (31,606,000)		$ (31,606,000)

[1] Rating trigger for CFC falls below A3/A-, while rating trigger for counterparty falls below Baa1/BBB+ by Moody’s or S&P, respectively.

[2] Rating trigger for CFC falls to or below Baa2/BBB, while rating trigger for counterparty falls to or below Ba2/BB+ by Moody’s or S&P, respectively.