Preferred Stock - Additional Information (Detail) - $ / shares	3 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Number of preferred stock shares authorized	50,000,000	50,000,000
Series K [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	23,000
Liquidation preference per share	$ 25,000
Preferred stock dividend rate fixed percentage	5.50%
Redemption period of preferred stock	90 days
Preferred stock redemption date	Oct. 15, 2023