Common Stock and Related Matters (Details) - USD ($) $ in Millions	12 Months Ended	30 Months Ended
Aug. 31, 2020	Aug. 31, 2019	Aug. 31, 2018	Aug. 31, 2020	Mar. 31, 2018
Common Stock [Roll Forward]
Common Stock, Balance beginning of period (in shares)	53,778,155
Common Stock, Balance end of period (in shares)	53,885,165	53,778,155		53,885,165
Common Stock, Amount (at par), Balance beginning of period	$ 0.5
Issuance of restricted stock grants, net of cancellations, Amount	32.7	$ 23.1	$ 23.7
Stock options exercised, Amount	0.1		$ 1.1
Common Stock, Amount (at par), Balance end of period	$ 0.5	$ 0.5		$ 0.5
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Remaining repurchased shares recorded as treasury stock	15,012,449	14,325,197		15,012,449
Treasury stock at original repurchase cost	$ 1,227.2	$ 1,156.0		$ 1,227.2
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock authorized (in shares)	50,000,000	50,000,000		50,000,000
Preferred stock, issued (in shares)	0	0		0
Preferred stock, outstanding (in shares)	0	0		0
March 2018 Repurchase Plan
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Shares authorized for repurchase (shares)					6,000,000
Reacquired shares of outstanding common stock	700,000			2,100,000
Maximum number of shares that may yet be purchased under the program (shares)	3,900,000			3,900,000
Amount
Common Stock [Roll Forward]
Common Stock, Balance beginning of period (in shares)	53,800,000	53,700,000	53,500,000
Issuance of restricted stock grants, net of cancellations (in shares)	100,000	100,000	200,000
Stock options exercised (in shares)	0		0
Common Stock, Balance end of period (in shares)	53,900,000	53,800,000	53,700,000	53,900,000
Common Stock, Amount (at par), Balance beginning of period	$ 0.5	$ 0.5	$ 0.5
Issuance of restricted stock grants, net of cancellations, Amount	0.0	0.0	0.0
Stock options exercised, Amount	0.0		0.0
Common Stock, Amount (at par), Balance end of period	$ 0.5	$ 0.5	$ 0.5	$ 0.5

Common Stock [Roll Forward]

No definition available.

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- Definition

Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.