Mandatorily Redeemable Preferred Stock - Additional Information (Detail) - Mandatorily Redeemable Preferred Stock [Member]	12 Months Ended
	Dec. 31, 2017 USD ($) vote / shares $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Preferred stock, shares authorized	350,000,000.0	350,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.01	$ 0.01
Preferred stock, shares issued	7,133	21,202
Preferred stock, shares outstanding	7,133	21,202
Preferred stock, dividend rate (as a percent)	8.00%	8.00%
Number of votes per share | vote / shares	1
Preferred stock, redemption value (in dollars per share) | $	$ 1,000	$ 1,000
Estimated fair value | $	$ 7,100,000	$ 21,300,000