Derivative Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Feb. 28, 2014		May 31, 2013
Counterparty Group
Derivative Financial Instruments
Notional amount	$ 6,576,953
Entity's required payment	(160,733)
Entity's amount collected	75,860
Entity's net payment	(84,873)
Counterparty Group | Mutual rating trigger falls to Baa1/BBB+
Derivative Financial Instruments
Notional amount	0	[1]
Entity's required payment	0	[1]
Entity's amount collected	0	[1]
Entity's net payment	0	[1]
Counterparty Group | Mutual rating trigger falls below Baa1/BBB+
Derivative Financial Instruments
Notional amount	6,576,953	[1]
Entity's required payment	(160,733)	[1]
Entity's amount collected	75,860	[1]
Entity's net payment	(84,873)	[1]
Interest rate swaps
Derivative Financial Instruments
Notional amount	$ 8,296,809		$ 8,788,329

[1] Stated senior unsecured credit ratings are for Moody’s Investors Service and Standard & Poor’s Corporation, respectively. Under these rating triggers, if the credit rating for either counterparty falls to the level specified in the agreement, the other counterparty may, but is not obligated to, terminate the agreement. If either counterparty terminates the agreement, a net payment may be due from one counterparty to the other based on the fair value, excluding credit risk, of the underlying derivative instrument.