Equity - Additional Information (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
Stockholder's Equity [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (82)	$ 832	$ (82)	$ 832	$ (114)	$ (147)	$ (214)	$ 8,544
Stockholders' Equity, Period Increase (Decrease)			(146,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1,157,894	1,627,138	1,157,894	1,627,138	918,428	1,303,882	1,506,576	1,505,853
Net income (loss)	241,600	119,726	(82,479)	167,704
Patronage capital retirement	(1,933)		$ (64,755)	(47,507)
Fiscal Period Duration			40 years
Expected Reclassification from Accumulated Other Comprehensive Income over Next Twelve Months Net of Tax			$ 1,000
Accumulated Net Investment Gain (Loss) Attributable to Parent
Stockholder's Equity [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	0	0	0	0	0	0	8,794
Patronage Capital Allocated
Stockholder's Equity [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	797,756	763,986	797,756	763,986	797,756	860,578	763,986	811,493
Patronage capital retirement			(62,822)	(47,507)
Members’ Capital Reserve
Stockholder's Equity [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	759,097	687,785	$ 759,097	687,785	759,097	759,097	687,785	687,785
Patronage Capital Retired
Stockholder's Equity [Line Items]
Fiscal Period Duration			39 years
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Stockholder's Equity [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	0	0	$ 0	0	0	0	(1,035)	(1,059)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax		1,035		1,059
Accumulated Defined Benefit Plans Adjustment, Net Transition Attributable to Parent
Stockholder's Equity [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(2,427)	(1,968)	(2,427)	(1,968)	(2,573)	(2,718)	(2,099)	(2,230)
AOCI, Derivative Qualifying as Hedge, Excluded Component, Parent [Member]
Stockholder's Equity [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 2,345	$ 2,800	2,345	$ 2,800	$ 2,459	$ 2,571	$ 2,920	$ 3,039
CFC | Patronage Capital Allocated
Stockholder's Equity [Line Items]
Patronage capital retirement			(62,822)
Net Income Allocations			$ 97,000
Patronage Refunds Percentage of Allocation of Net Earnings			50.00%
CFC | Retained Earnings Allocation of Fifty Percent of Prior Year Patronage Capital Allocation [Member]
Stockholder's Equity [Line Items]
Patronage capital retirement			$ (48,000)
CFC | Retained Earnings Allocation Held for Twenty Five Years [Member]
Stockholder's Equity [Line Items]
Patronage capital retirement			(15,000)
CFC | Cooperative educational fund
Stockholder's Equity [Line Items]
Net Income Allocations			1,000
CFC | Members’ Capital Reserve
Stockholder's Equity [Line Items]
Net Income Allocations			$ 71,000