Investment Securities - Amortized cost and Fair Value of Equity Securities and Corresponding Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Nov. 30, 2019	May 31, 2019
Debt and Equity Securities [Line Items]
Equity securities	$ 63,809	$ 87,533
Preferred Stock
Debt and Equity Securities [Line Items]
Equity securities	57,873	82,445
Common Stock
Debt and Equity Securities [Line Items]
Equity securities	$ 5,936	$ 5,088

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

Name:	us-gaap_EquitySecuritiesFvNi
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant