Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	49 Months Ended
Nov. 30, 2019	Aug. 31, 2019	Nov. 30, 2018	Nov. 30, 2019	Aug. 31, 2019	Mar. 21, 2018	Jul. 22, 2015
Stockholders' Equity Attributable to Parent [Abstract]
Authorized amount of stock to be repurchased	$ 100.0			$ 100.0		$ 50.0	$ 50.0
Total current Board authorizations	(7.1)	$ (12.6)	$ 0.0	(19.6)	$ (66.8)
Less repurchases made under these authorizations	$ 33.2			$ 33.2

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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