Fair Value Measurement Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Nov. 30, 2019	May 31, 2019	Nov. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 114,033	$ 177,922
Cash and Cash Equivalents, Fair Value Disclosure	114,033	177,922
Restricted cash	10,638	8,282
Equity securities	63,809	87,533
Debt Securities, Held-to-maturity	573,547	565,444
Debt Securities, Held-to-maturity, Fair Value	583,439	570,549
Deferred Compensation Plan Assets	5,682	4,984
Loans and Leases Receivable Commercial, Net of Allowance	26,421,661	25,899,369	$ 25,277,271
Loans Receivable, Fair Value Disclosure	28,008,555	25,743,503
Accrued interest receivable	130,950	133,605
Debt service reserve restricted funds	17,151	17,151
Derivative assets	53,174	41,179
Short-term borrowings	4,789,024	3,607,726
Short-term Debt, Fair Value	4,789,768	3,608,259
Long-term debt	18,434,451	19,210,793
Long-term Debt, Fair Value	19,783,264	20,147,183
Accrued interest payable	148,875	158,997
Guaranty Liabilities Contingent and Noncontingent	13,489	13,666
Guarantees, Fair Value Disclosure	13,763	13,307
Derivative liabilities	591,027	391,724
Subordinated deferrable debt	986,026	986,020
Subordinated Debt Obligations, Fair Value Disclosure	1,052,215	1,004,707
Members Subordinated Certificates	1,355,052	1,357,129
Members Subordinated Certificates, At Fair Value	1,355,052	1,357,129
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	114,033	177,922
Restricted cash	10,638	8,282
Equity securities	63,809	87,533
Deferred Compensation Plan Assets	5,682	4,984
Accrued interest receivable	0
Debt service reserve restricted funds	17,151	17,151
Short-term Debt, Fair Value	0
Accrued interest payable	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Held-to-maturity, Fair Value	583,439	570,549
Accrued interest receivable	130,950	133,605
Derivative assets	53,174	41,179
Short-term Debt, Fair Value	4,639,768	3,608,259
Long-term Debt, Fair Value	11,153,207	11,482,715
Accrued interest payable	148,875	158,997
Derivative liabilities	591,027	391,724
Subordinated Debt Obligations, Fair Value Disclosure	1,052,215	1,004,707
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable, Fair Value Disclosure	28,008,555	25,743,503
Short-term Debt, Fair Value	150,000	0
Long-term Debt, Fair Value	8,630,057	8,664,468
Guarantees, Fair Value Disclosure	13,763	13,307
Members Subordinated Certificates, At Fair Value	$ 1,355,052	$ 1,357,129