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	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Deferred stock units held by directors	228,839	197,217	169,027
Common stock, shares issued under dividend reinvestment program	1,688	1,401	1,070