Income Taxes - Summary of Taxability of Cash Distributions Paid on Common Shares (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Ordinary income	$ 2.97	$ 2.98	$ 2.50
Capital gains	0.03	0.17	0.23
Return of capital	0.15
Total	$ 3.15	$ 3.15	$ 2.73