Consolidated Statements of Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	May 31, 2017	May 31, 2016	May 31, 2015
Statement of Stockholders' Equity [Abstract]
Dividends declared per common share (in dollars per share)	$ 0.70	$ 0.62	$ 0.54
Dividends declared per preferred share (in dollars per share)	$ 0.10	$ 0.10	$ 0.10