Loans and Commitments (Details 11) - USD ($) $ in Thousands		Feb. 29, 2016		May. 31, 2015		Feb. 28, 2015
Financing Receivable, Modifications [Line Items]
Total loans outstanding		$ 23,134,215	[1],[2]	$ 21,459,220	[1],[2]	$ 21,202,399
Other Commitment		3,000		2,000
Loans outstanding		$ 13,081		4,221
Unadvanced commitments				$ 0
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage		100.00%		100.00%
Nonperforming Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Loans outstanding		$ 3,506
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage		0.02%
Non-performing loans
Financing Receivable, Modifications [Line Items]
Loans outstanding		$ 2,358		$ 0
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage		0.01%
Performing Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Total loans outstanding		$ 13,933		$ 11,736
Performing TDR Loans As Percentage of Total Loans		0.06%		0.05%
Troubled Debt Restructuring Financing Receivable [Member]
Financing Receivable, Modifications [Line Items]
Total loans outstanding		$ 17,439		$ 11,736
Loans and Leases Receivable Commercial, Net of Deferred Income, Percentage		0.08%		0.05%
Line of credit loans
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	$ 1,113,990		$ 1,038,210
Long-term fixed-rate loans
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	21,127,042		19,543,274
Long-term variable-rate loans
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	718,193		698,495
CFC
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	22,079,012		20,341,990
CFC | Performing Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Loans outstanding	[3]	6,716		7,221
RTFC
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	357,967		385,709
Loans outstanding		13,081		4,221
RTFC | Non-performing loans
Financing Receivable, Modifications [Line Items]
Loans outstanding		2,358
RTFC | Performing Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Loans outstanding		7,217		4,221
RTFC | Long-term variable-rate loans | Nonperforming Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Loans outstanding		3,506
RTFC | Long-term variable-rate loans | Non-performing loans
Financing Receivable, Modifications [Line Items]
Loans outstanding				0
NCSC
Financing Receivable, Modifications [Line Items]
Total loans outstanding	[1]	697,236		731,521
NCSC | Performing Troubled Debt Restructuring loans [Member]
Financing Receivable, Modifications [Line Items]
Loans outstanding		$ 0		$ 294

[1] Includes nonperforming and restructured loans.

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

[3] A borrower in this category also had a line of credit loan outstanding that was classified as performing as of February 29, 2016 and May 31, 2015. Unadvanced commitments related to this line of credit loan totaled $3 million and $2 million as of February 29, 2016 and May 31, 2015, respectively.