Preferred and Common Stockholder's Equity - Schedule of Stock Repurchases (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Dec. 31, 2018	Dec. 31, 2018
Stockholders Equity [Line Items]
Shares purchased and retired		1.6
Common stock and additional paid-in-capital reductions	$ 16.9	$ 16.9
Charges to retained earnings	$ 33.1	$ 33.1
Common Stock [Member]
Stockholders Equity [Line Items]
Shares purchased and retired	1.6	1.6