Fair Value Measurement Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	May 31, 2018	May 31, 2017	May 31, 2016	May 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 230,999	$ 166,615	$ 204,540	$ 248,836
Cash and Cash Equivalents, Fair Value Disclosure	230,999	166,615
Restricted cash	7,825	21,806
Time deposits	101,000	226,000
Time Deposits, At Fair Value	101,000	226,000
Available for sale, at fair value	89,332	92,554
Debt Securities, Held-to-maturity	519,519	0
Debt Securities, Held-to-maturity, Fair Value	515,546
Deferred Compensation Plan Assets	5,194	4,693
Loans to members, net	25,159,807	24,329,668
Loans Receivable, Fair Value Disclosure	24,167,886	24,182,724
Accrued interest receivable	127,442	111,493
Debt service reserve restricted funds	17,151	17,151
Derivative assets	244,526	49,481
Short-term Debt	3,795,910	3,342,900
Short-term Debt, Fair Value	3,795,799	3,342,990
Long-term Debt	18,714,960	17,955,594
Long-term Debt, Fair Value	18,909,276	18,744,331
Accrued interest payable	149,284	137,476
Guaranty Liabilities Contingent and Noncontingent	10,589	15,241
Guarantees, Fair Value Disclosure	10,454	16,204
Derivative liabilities	275,932	385,337
Subordinated deferrable debt	742,410	742,274
Subordinated Debt Obligations, Fair Value Disclosure	766,088	788,079
Members Subordinated Certificates, Total	1,379,982	1,419,025
Members Subordinated Certificates, At Fair Value	1,380,004	1,419,048
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	230,999	166,615
Restricted cash	7,825	21,806
Available for sale, at fair value	89,332
Deferred Compensation Plan Assets	5,194	4,693
Debt service reserve restricted funds	17,151	17,151
Short-term Debt, Fair Value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Time Deposits, At Fair Value	101,000	226,000
Debt Securities, Held-to-maturity, Fair Value	515,546
Accrued interest receivable	127,442	111,493
Derivative assets	244,526	49,481
Short-term Debt, Fair Value	3,695,799	3,342,990
Long-term Debt, Fair Value	11,373,216	11,215,290
Accrued interest payable	149,284	137,476
Derivative liabilities	275,932	385,337
Subordinated Debt Obligations, Fair Value Disclosure	766,088	788,079
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure	24,167,886	24,182,724
Short-term Debt, Fair Value	100,000
Long-term Debt, Fair Value	7,536,060	7,529,041
Guarantees, Fair Value Disclosure	10,454	16,204
Members Subordinated Certificates, At Fair Value	1,380,004	1,419,048
Recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Plan Assets	5,194	4,693
Derivative assets	244,526	49,481
Derivative liabilities	275,932	385,337
Recurring basis | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred Compensation Plan Assets		4,693
Recurring basis | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	244,526	49,481
Derivative liabilities	$ 275,932	$ 385,337