Impaired Loans - Average Recorded Investment and Interest Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Average recorded investment
Total impaired loans	$ 11,179	$ 12,184	$ 11,484	$ 12,369
Interest income recognized
Interest impaired loans	212	211	418	429
CFC
Average recorded investment
Total impaired loans	5,755	6,261	5,999	6,383
Interest income recognized
Interest impaired loans	144	137	281	279
RTFC
Average recorded investment
Total impaired loans	5,424	5,923	5,485	5,986
Interest income recognized
Interest impaired loans	$ 100	$ 74	$ 137	$ 150