Loans and Commitments (Details 10) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Feb. 29, 2016		Feb. 28, 2015	Feb. 29, 2016		Feb. 28, 2015	May. 31, 2015
Recorded investment in individually-impaired loans and the related specific valuation allowance
With no specific allowance recorded		$ 6,716			$ 6,716			$ 7,515
Impaired Financing Receivable, with Related Allowance, Recorded Investment		13,081			13,081			4,221
Total impaired loans		19,797			19,797			11,736
Related allowance		4,554			4,554			$ 395
Average recorded investment
Total impaired loans		20,078		$ 9,113	15,976		$ 9,332
Interest income recognized
Interest impaired loans		243		0	402		10
Interest income reduction as a result of holding loans on non-accrual status
Interest income reduced		$ 33		155	$ 257		470
Loans receivable as a percentage of total loan (percent)		100.00%			100.00%			100.00%
Total loans outstanding		$ 23,134,215	[1],[2]	21,202,399	$ 23,134,215	[1],[2]	21,202,399	$ 21,459,220	[1],[2]
Non-performing loans
Recorded investment in individually-impaired loans and the related specific valuation allowance
Impaired Financing Receivable, with Related Allowance, Recorded Investment		2,358			2,358			0
Interest income reduction as a result of holding loans on non-accrual status
Interest income reduced		2		23	14		74
Performing Troubled Debt Restructuring loans [Member]
Interest income reduction as a result of holding loans on non-accrual status
Interest income reduced		0		132	166		396
Total loans outstanding		13,933			13,933			11,736
Restructured loans
Interest income recognized
Interest impaired loans					0
Nonperforming Troubled Debt Restructuring loans [Member]
Recorded investment in individually-impaired loans and the related specific valuation allowance
Impaired Financing Receivable, with Related Allowance, Recorded Investment		3,506			3,506
Interest income reduction as a result of holding loans on non-accrual status
Interest income reduced		$ 31		0	$ 77		0
Loans receivable as a percentage of total loan (percent)		0.02%			0.02%
CFC
Interest income reduction as a result of holding loans on non-accrual status
Total loans outstanding	[1]	$ 22,079,012			$ 22,079,012			20,341,990
CFC | Distribution
Recorded investment in individually-impaired loans and the related specific valuation allowance
With no specific allowance recorded		6,716			6,716			7,221
Average recorded investment
Total impaired loans		6,716		7,221	6,884		7,342
Interest income recognized
Interest impaired loans		130		0	260		0
Interest income reduction as a result of holding loans on non-accrual status
Total loans outstanding	[1]	17,700,859			17,700,859			16,095,043
CFC | Power supply
Interest income reduction as a result of holding loans on non-accrual status
Total loans outstanding	[1]	4,322,069			4,322,069			4,181,481
NCSC
Recorded investment in individually-impaired loans and the related specific valuation allowance
With no specific allowance recorded		0			0			294
Average recorded investment
Total impaired loans		0		312	0		334
Interest income recognized
Interest impaired loans		0		0			10
Interest income reduction as a result of holding loans on non-accrual status
Total loans outstanding	[1]	697,236			697,236			731,521
RTFC
Recorded investment in individually-impaired loans and the related specific valuation allowance
Impaired Financing Receivable, with Related Allowance, Recorded Investment		13,081			13,081			4,221
Related allowance		4,554			4,554			395
Average recorded investment
Total impaired loans		13,362		1,580	9,092		$ 1,656
Interest income recognized
Interest impaired loans		113		$ 0	142
Interest income reduction as a result of holding loans on non-accrual status
Total loans outstanding	[1]	357,967			357,967			$ 385,709
RTFC | Non-performing loans
Recorded investment in individually-impaired loans and the related specific valuation allowance
Impaired Financing Receivable, with Related Allowance, Recorded Investment		$ 2,358			$ 2,358

[1] Includes nonperforming and restructured loans.

[2] Represents the unpaid principal balance excluding deferred loan origination costs.