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, 2017	$ 45.2	$ 460.8	$ 727.9	$ (279.8)	$ (39.9)	$ 914.2
Increase (Decrease) in Stockholders' Equity
Net income			10.6			10.6
Cash dividends			(2.6)			(2.6)
Stock option activity		(0.6)		6.6		6.0
Restricted stock activity	0.1	0.1		0.6		0.8
Repurchase of shares				(5.2)		(5.2)
Other comprehensive income (loss), net of tax					0.9	0.9
Balance at Aug. 31, 2017	45.3	460.3	735.9	(277.8)	(39.0)	924.7
Balance at May. 31, 2017	45.2	460.8	727.9	(279.8)	(39.9)	914.2
Increase (Decrease) in Stockholders' Equity
Net income						3.6
Other comprehensive income (loss), net of tax						4.3
Balance at Feb. 28, 2018	45.3	464.1	723.8	(282.4)	(35.6)	915.2
Balance at Aug. 31, 2017	45.3	460.3	735.9	(277.8)	(39.0)	924.7
Increase (Decrease) in Stockholders' Equity
Net income			(22.5)			(22.5)
Cash dividends			(2.6)			(2.6)
Stock option activity		0.7		2.6		3.3
Restricted stock activity		1.3		0.2		1.5
Other comprehensive income (loss), net of tax					2.2	2.2
Balance at Nov. 30, 2017	45.3	462.3	710.8	(275.0)	(36.8)	906.6
Increase (Decrease) in Stockholders' Equity
Net income			15.5			15.5
Cash dividends			(2.5)			(2.5)
Stock option activity		(0.2)		0.3		0.1
Restricted stock activity		2.0		0.2		2.2
Repurchase of shares				(7.9)		(7.9)
Other comprehensive income (loss), net of tax					1.2	1.2
Balance at Feb. 28, 2018	45.3	464.1	723.8	(282.4)	(35.6)	915.2
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment upon adoption of ASC 606 on June 1, 2018 | ASU 2014-09			(20.4)			(20.4)
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			(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
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			(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			(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