Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Millions	Common Stock	Capital Surplus	Retained Earnings	Treasury Stock	Accumulated Other Comprehensive Income (Loss)	Total
Balance at May. 31, 2018	$ 45.3	$ 470.5	$ 733.2	$ (280.7)	$ (32.0)	$ 936.3
Increase (Decrease) in Stockholders' Equity
Net income			15.1			15.1
Cash dividends ($0.075 per share)			(2.7)			(2.7)
Stock option activity		0.7		2.2		2.9
Restricted stock activity		(1.4)		(0.5)		(1.9)
Other comprehensive income (loss), net of tax					(0.2)	(0.2)
Balance at Aug. 31, 2018	45.3	469.8	725.2	(279.0)	(32.2)	929.1
Balance at May. 31, 2018	45.3	470.5	733.2	(280.7)	(32.0)	936.3
Increase (Decrease) in Stockholders' Equity
Net income						(15.3)
Other comprehensive income (loss), net of tax						0.2
Balance at Feb. 28, 2019	45.3	474.8	689.6	(278.5)	(31.8)	899.4
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment upon adoption of ASC | ASC 606			(20.4)			(20.4)
Balance at Aug. 31, 2018	45.3	469.8	725.2	(279.0)	(32.2)	929.1
Increase (Decrease) in Stockholders' Equity
Net income			7.0			7.0
Cash dividends ($0.075 per share)			(2.6)			(2.6)
Stock option activity		0.9		1.6		2.5
Restricted stock activity		0.1		(0.1)
Other comprehensive income (loss), net of tax					(0.4)	(0.4)
Balance at Nov. 30, 2018	45.3	470.8	729.6	(277.5)	(32.6)	935.6
Increase (Decrease) in Stockholders' Equity
Net income			(37.4)			(37.4)
Cash dividends ($0.075 per share)			(2.6)			(2.6)
Stock option activity		1.0				1.0
Restricted stock activity		3.0		(0.2)		2.8
Repurchase of shares				(0.8)		(0.8)
Other comprehensive income (loss), net of tax					0.8	0.8
Balance at Feb. 28, 2019	45.3	474.8	689.6	(278.5)	(31.8)	899.4
Balance at May. 31, 2019	45.3	479.4	709.8	(287.7)	(40.9)	905.9
Increase (Decrease) in Stockholders' Equity
Net income			4.4			4.4
Cash dividends ($0.075 per share)			(2.9)			(2.9)
Stock option activity		0.9		1.8		2.7
Restricted stock activity		(4.3)		1.1		(3.2)
Other comprehensive income (loss), net of tax					0.1	0.1
Balance at Aug. 31, 2019	45.3	476.0	713.8	(284.8)	(40.8)	909.5
Balance at May. 31, 2019	45.3	479.4	709.8	(287.7)	(40.9)	905.9
Increase (Decrease) in Stockholders' Equity
Net income						20.9
Other comprehensive income (loss), net of tax						1.0
Balance at Feb. 29, 2020	45.3	481.5	725.1	(282.6)	(39.9)	929.4
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment upon adoption of ASC | ASC 842			2.5			2.5
Balance at Aug. 31, 2019	45.3	476.0	713.8	(284.8)	(40.8)	909.5
Increase (Decrease) in Stockholders' Equity
Net income			14.2			14.2
Cash dividends ($0.075 per share)			(2.6)			(2.6)
Stock option activity		0.9		1.2		2.1
Restricted stock activity		1.8				1.8
Repurchase of shares				(4.1)		(4.1)
Other comprehensive income (loss), net of tax					0.8	0.8
Balance at Nov. 30, 2019	45.3	478.7	725.4	(287.7)	(40.0)	921.7
Increase (Decrease) in Stockholders' Equity
Net income			2.3			2.3
Cash dividends ($0.075 per share)			(2.6)			(2.6)
Stock option activity		0.5		5.3		5.8
Restricted stock activity		2.3		(0.2)		2.1
Other comprehensive income (loss), net of tax					0.1	0.1
Balance at Feb. 29, 2020	$ 45.3	$ 481.5	$ 725.1	$ (282.6)	$ (39.9)	$ 929.4