Variable Interest Entities Additional Information (Details) $ in Millions	12 Months Ended
	May 31, 2018 USD ($) director vote
Variable Interest Entity [Line Items]
Number of States in which Electric and Telecommunications Members are Located	50
Consolidated variable interest entities
Variable Interest Entity [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted | $	$ 28
RTFC | Consolidated variable interest entities
Variable Interest Entity [Line Items]
Number of votes by each member for election of directors	1
NCSC | Consolidated variable interest entities
Variable Interest Entity [Line Items]
Number of directors for whom nomination process is controlled | director	1
Number of votes by each member for election of directors	1