Equity - Schedule of Movement in Outstanding Shares of Class A Common Stock, Restricted Class A Common Stock and Deferred Stock Units (Parenthetical) (Detail) - Class A Common Stock [Member] - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Class of Stock [Line Items]
Deferred stock units held by directors	205,088	175,442
Common stock, shares issued under dividend reinvestment program	455	270