Equity (Details) - USD ($) $ in Thousands		1 Months Ended		3 Months Ended		9 Months Ended
		Jul. 31, 2015	Jul. 31, 2014	Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015	Nov. 30, 2015	May. 31, 2015	Nov. 30, 2014	May. 31, 2014
Equity
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax					$ (1,091)		$ (1,091)
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments	[1]			$ 1,262,992		$ 1,262,992			$ 1,167,319
Accumulated Other Comprehensive Income (Loss), Net of Tax				4,114	7,119	4,114	7,119	$ 6,585	4,080	$ 6,773	$ 3,649
Stockholders' Equity, Period Increase (Decrease)						(195,000)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				716,496	884,504	716,496	884,504		911,786		970,374
Net income				(174,382)	(34,196)	(155,775)	(49,496)
Retirement/allocation of net earnings authorized						(39,384)	(39,662)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax				(2,297)	1,644	589	5,246
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax				43	29	131	29
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(217)	(236)	(686)	(714)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				(2,471)	346	34	3,470
Prior Years Cumulative Derivative Forward Value and Foreign Currency Adjustments				287,077		287,077			172,412
Derivative Forward Value Gain (Loss)	[2]			289,498		289,498			114,665
Total Cumulative Derivative Forward Value and Foreign Currency Adjustments				576,575		576,575			287,077
Retained equity				686,417		686,417			880,242
Stockholders' Equity Attributable to Parent				690,531		690,531			884,322
Noncontrolling interests				25,965		25,965			27,464
Retained Earnings Appropriated Membership Fees [Member]
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments				972		972			976
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Equity
Accumulated Other Comprehensive Income (Loss), Net of Tax				4,523	4,885	4,523	4,885	6,820	3,934	3,241	(361)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax				(2,297)	1,644	589	5,246
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				(2,297)	1,644	589	5,246
Cooperative educational fund
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments				1,123		1,123			1,767
Membership Fees and Educational Fund
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments				2,095		2,095			2,743
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				2,095	2,113	2,095	2,113		2,743		2,751
Patronage Capital Allocated
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments				501,302		501,302			501,731
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				629,596	590,677	629,596	590,677		668,980		630,340
Retirement/allocation of net earnings authorized						(39,384)	(39,662)
Members’ Capital Reserve
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments				629,596		629,596			668,980
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest				501,302	485,448	501,302	485,448		501,731		485,447
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Equity
Accumulated Other Comprehensive Income (Loss), Net of Tax				4,685	5,606	4,685	5,606	4,902	5,371	5,842	6,320
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(217)	(236)	(686)	(714)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				(217)	(236)	(686)	(714)
Accumulated Unrealized Losses on Foreclosed Assets [Member]
Equity
Accumulated Other Comprehensive Income (Loss), Net of Tax				(4,248)	(2,310)	(4,248)	(2,310)	(4,248)	(4,248)	$ (2,310)	$ (2,310)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				0	0	0	0
Accumulated Defined Benefit Plans Adjustment, Net Transition Attributable to Parent [Member]
Equity
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax					(1,091)		(1,091)
Accumulated Other Comprehensive Income (Loss), Net of Tax				(846)	(1,062)	(846)	(1,062)	$ (889)	(977)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax				43	29	131	29
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				43	$ (1,062)	131	$ (1,062)
Retained Earnings Excluding Derivative Forward Unappropriated [Member]
Equity
Retained Earnings before Derivate Forward Value and Foreign Currency Adjustments	[1]			$ 129,999		$ 129,999			$ (6,135)
CFC | Cooperative educational fund | Authorized Allocations
Equity
Allocation of net earnings		$ 1,000
CFC | Patronage Capital Allocated | Authorized Allocations
Equity
Allocation of net earnings		78,000
Retirement of allocated net earnings (percentage)			50.00%
CFC | Members’ Capital Reserve
Equity
Allocation of net earnings		$ 16,000

[1] Excludes derivative forward value.

[2] Represents the derivative forward value income (loss) recorded by CFC for the year-to-date period.