Allowance for Loan Losses - Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018	Aug. 31, 2019	May 31, 2019	Aug. 31, 2018	May 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan loss	$ 16,520	$ 16,904	$ 16,520	$ 16,904	$ 17,565	$ 17,535	$ 18,692	$ 18,801
Provision for Loan and Lease Losses	1,045	1,788	(1,015)	1,897
CFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan loss	13,076	12,174	13,076	12,174	12,962	13,120	12,508	12,300
Provision for Loan and Lease Losses	(114)	334	44	(126)
NCSC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan loss	810	1,969	810	1,969	2,077	2,007	2,012	2,082
Provision for Loan and Lease Losses	1,267	43	1,197	113
RTFC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan loss	2,634	2,761	2,634	2,761	$ 2,526	$ 2,408	$ 4,172	$ 4,419
Provision for Loan and Lease Losses	$ 108	$ (1,411)	$ 226	$ (1,658)