Guarantees - Additional Information (Details) - USD ($) $ in Thousands		6 Months Ended
		Nov. 30, 2019	May 31, 2019
Guarantees
Guarantor obligations		$ 794,723	$ 837,435
Guarantee Obligations Unsecured		$ 352,000	$ 374,000
Guarantee Obligations Unsecured Commitment as Percentage of Total Commitment		44.00%	45.00%
Guarantee Liability Recorded		$ 13,000	$ 14,000
Guaranty Liabilities Contingent		1,000	1,000
Guaranty Liabilities		12,000	13,000
Long-term variable-rate loans
Guarantees
Guarantee Obligations Liquidity Provided to Member Carrying Value		245,000
Long-term tax-exempt bonds
Guarantees
Guarantor obligations	[1]	308,750	312,190
Financial Guarantee | Long-term fixed-rate bonds
Guarantees
Guarantor obligations		64,000
Financial Guarantee | Long Term Fixed Rate Loans
Guarantees
Guarantor Obligations, Maximum Exposure, Undiscounted		89,000
Financial Guarantee | Long-term variable-rate loans
Guarantees
Guarantor obligations		245,000	247,000
Letters of credit
Guarantees
Guarantor obligations	[2]	339,241	379,001
Guarantee Obligations Secured		109,000	126,000
Master Letter of Credit | Master Letter of Credit
Guarantees
Letter of Credit Facility Maximum Additional Amount Potentially Required to be Issued		54,000
Other guarantees
Guarantees
Guarantor obligations		146,732	146,244
Guarantor Obligations, Maximum Exposure, Undiscounted		147,000	147,000
Other secured guarantees
Guarantees
Guarantor Obligations, Maximum Exposure, Undiscounted		$ 25,000	$ 25,000

[1] Represents the outstanding principal amount of long-term fixed-rate and variable-rate guaranteed bonds.

[2] Reflects our maximum potential exposure for letters of credit.