Treasury Stock - Schedule of repurchase of common stock (Details) - USD ($) $ in Millions	3 Months Ended	49 Months Ended
Aug. 31, 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	(12.6)	(59.8)
Less repurchases made under these authorizations	$ 40.2	$ 40.2

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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