Common Stock and Related Matters - Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Aug. 31, 2020	May 31, 2020	Feb. 29, 2020	Nov. 30, 2019	Aug. 31, 2019	May 31, 2019	Feb. 28, 2019	Nov. 30, 2018	Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018
Earnings per share:
Net income									$ 248.3	$ 330.4	$ 349.6
Basic weighted average shares outstanding (in shares)									39,500,000	39,700,000	40,900,000
Common stock equivalents (in shares)									100,000	100,000	100,000
Diluted weighted average number of shares outstanding (in shares)									39,600,000	39,800,000	41,000,000.0
Basic earnings per share (in dollars per share)	$ 1.88	$ 1.53	$ 1.45	$ 1.44	$ 2.43	$ 2.23	$ 1.68	$ 1.99	$ 6.29	$ 8.32	$ 8.54
Diluted earnings per share (in dollars per share)	$ 1.87	$ 1.52	$ 1.44	$ 1.44	$ 2.42	$ 2.22	$ 1.67	$ 1.98	$ 6.27	$ 8.29	$ 8.52
Stock Options
Earnings per share:
Stock options excluded from diluted earnings per share (in shares)									598,000	300,000	179,000
Restricted Stock
Earnings per share:
Stock options excluded from diluted earnings per share (in shares)									213,000	160,000	227,000
Performance stock units
Earnings per share:
Stock options excluded from diluted earnings per share (in shares)									0	0	0