Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Feb. 28, 2014	May 31, 2013
Interest rate swaps
Derivative Financial Instruments
Notional amount	$ 8,296,809	$ 8,788,329
Weighted-average rate paid (as a percent)	2.42%	2.49%
Weighted-average rate received (as a percent)	1.50%	2.00%
Pay fixed-receive variable
Derivative Financial Instruments
Notional amount	5,172,809	5,287,889
Weighted-average rate paid (as a percent)	3.36%	3.39%
Weighted-average rate received (as a percent)	0.22%	0.26%
Pay variable-receive fixed
Derivative Financial Instruments
Notional amount	$ 3,124,000	$ 3,500,440
Weighted-average rate paid (as a percent)	0.86%	1.12%
Weighted-average rate received (as a percent)	3.62%	4.62%