Members' Subordinated Certificates Maturities (Details) - USD ($) $ in Thousands		12 Months Ended
		May 31, 2018	May 31, 2017
Amount Maturing
Total long-term debt		$ 742,410	$ 742,274
Members' Certificates, exclusive of certificates amortized annually
Amount Maturing
2019	[1]	9,818
2020	[1]	13,360
2021	[1]	42,932
2022	[1]	14,381
2023	[1]	30,908
Thereafter	[1]	1,268,251
Total long-term debt	[1]	$ 1,379,650
Weighted-Average Interest Rate
2019 (as a percent)		2.82%
2020 (as a percent)		2.85%
2021 (as a percent)		3.64%
2022 (as a percent)		2.92%
2023 (as a percent)		2.70%
Thereafter (as a percent)		4.27%
Total (as a percent)		4.18%
Loan Subordinated Certificates
Amount Maturing
Total long-term debt		$ 463,058	$ 482,441
Other information
Payments not received on certificates subscribed and unissued		0
Average amortization of debt		$ 16,000
Amortization as a percentage of amortizing loan subordinated debt outstanding		6.00%
Amortizing Member Loan Subordinated Certificates
Amount Maturing
Total long-term debt		$ 274,000

[1] Amortization payments on these certificates totaled $16 million in fiscal year 2018 and represented 6% of amortizing loan subordinated certificates outstanding.