Summary of Significant Accounting Policies - Interest Expense (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Interest expense on debt:
Interest expense	[1],[2]	$ 207,871	$ 204,166	$ 421,142	$ 414,397
Short-term borrowings
Interest expense on debt:
Interest expense	[1],[2]	22,112	22,619	44,934	42,038
Medium-term notes
Interest expense on debt:
Interest expense	[1],[2]	31,440	33,816	63,516	66,226
Collateral trust bonds
Interest expense on debt:
Interest expense	[1],[2]	64,523	68,934	129,904	146,639
Guaranteed Underwriter Program notes payable
Interest expense on debt:
Interest expense	[1],[2]	39,786	35,014	80,219	70,348
Farmer Mac notes payable
Interest expense on debt:
Interest expense	[1],[2]	22,654	19,697	47,728	40,808
Other notes payable
Interest expense on debt:
Interest expense	[1],[2]	230	322	484	644
Subordinated deferrable debt
Interest expense on debt:
Interest expense	[1],[2]	12,884	9,417	25,766	18,834
Subordinated certificates
Interest expense on debt:
Interest expense	[1],[2]	$ 14,242	$ 14,347	$ 28,591	$ 28,860

[1] Includes amortization of debt discounts and debt issuance costs, which are generally deferred and recognized as interest expense using the effective interest method. Issuance costs related to dealer commercial paper, however, are recognized as interest expense immediately as incurred.

[2] Includes fees related to funding arrangements, such as up-front fees paid to banks participating in our committed bank revolving line of credit agreements. Depending on the nature of the fee, amounts may be deferred and recognized as interest expense ratably over the term of the arrangement or recognized immediately as incurred.