Redeemable Preferred Stock - Additional Information (Detail) - Redeemable Preferred Stock [Member] - USD ($) $ / shares in Units, $ in Millions		1 Months Ended
	Jun. 28, 2013	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Preferred stock sold	4,000
Preferred stock, par value	$ 5,000
Preferred stock dividend rate	9.50%
Redeemable Preferred Stock on Balance Sheet			$ 20	$ 20
Call Option [Member]
Class of Stock [Line Items]
Preferred stock redemption period	5 years
Put Option [Member]
Class of Stock [Line Items]
Preferred stock redemption period	10 years
Subsequent Event [Member]
Class of Stock [Line Items]
Percentage of outstanding redeemable preferred stock		100.00%