Derivative Instruments and Hedging Activities Additional Information (Details) - USD ($) $ in Thousands		12 Months Ended
		May 31, 2018	May 31, 2017
Derivative [Line Items]
Long-term Debt		$ 18,714,960	$ 17,955,594
Derivative Liability, Notional Amount		5,903,182	7,037,276
Treasury Lock
Derivative [Line Items]
Derivative Liability, Notional Amount		100,000	$ 0
Interest Rate Contracts and Treasury Lock [Member]
Derivative [Line Items]
Derivative, Notional Amount	[1]	200,000
Assets Needed for Immediate Settlement, Aggregate Fair Value		1,000
Interest rate swaps
Derivative [Line Items]
Derivative Liability, Notional Amount		5,803,182
Derivative, Net Liability Position, Aggregate Fair Value		$ 81,000
Percentage of concentration		24.00%	23.00%
Guaranteed Underwriter Program Notes Payable
Derivative [Line Items]
Long-term Debt		$ 4,856,143	$ 4,985,484
Designated as Hedging Instrument | Treasury Lock
Derivative [Line Items]
Derivative Liability, Notional Amount		100,000
Designated as Hedging Instrument | Guaranteed Underwriter Program Notes Payable
Derivative [Line Items]
Long-term Debt		$ 100,000

[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.