Income Taxes - Schedule of Per Share Distributions to Stockholders (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Common share distribution	$ 0.950000	$ 0.900000	$ 0.870000
Ordinary income	0.655535	0.619368	0.769535
Capital gains	0.021022	0.102552
Unrecaptured Sec. 1250 gain	0.004647	0.045432
Return of capital	$ 0.273443	$ 0.178080	$ 0.100465