Dividends/Distributions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Dividends [Line Items]
Distributions paid, per share	$ 0.83	$ 0.76	$ 2.49	$ 2.26
Distributions paid, preferred stockholders	$ 91	$ 91	$ 273	$ 273
Distributions paid, preferred stockholders, per share	$ 15.95	$ 15.95	$ 47.85	$ 47.85
Taxable Income Distribution [Member]
Dividends [Line Items]
Distributions paid	$ 81,408	$ 73,938	$ 243,928	$ 219,584
Distributions paid, per share	$ 0.83	$ 0.76	$ 2.49	$ 2.26