Stock Transactions and Unit Redemptions - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Aug. 01, 2018	Jun. 30, 2018	Mar. 12, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Number of authorized classes of common stock	2		2
Share repurchase and redemption unit remaining authorized amount			$ 165,500,000
Class A Common Stock [Member]
Class Of Stock [Line Items]
Common stock voting rights description			Each share of Class A common stock is entitled to one vote
Common stock, shares authorized	1,000,000,000		1,000,000,000	1,000,000,000
Common stock, par value	$ 0.01		$ 0.01	$ 0.01
Common stock, shares outstanding	154,989,126	156,679,527	154,989,126	156,916,336	138,938,972		138,921,533		138,593,786
Stock repurchased during period	2,279,373		3,892,405
Stock repurchased average price per share	$ 8.81	$ 8.61	$ 8.74	$ 9.73
Share repurchase and redemption unit remaining authorized amount	$ 165,530,000	$ 185,618,000	$ 165,530,000	$ 199,514,000
Class A Common Stock [Member] | Limited Partnership Interests or Other Equity Interests in Subsidiaries [Member] | Affiliated Persons or Entities [Member]
Class Of Stock [Line Items]
Stock repurchases and redeemed or repurchases authorized amount								$ 100,000,000.0
Class A Common Stock [Member] | Maximum [Member] | Limited Partnership Interests or Other Equity Interests in Subsidiaries [Member] | Affiliated Persons or Entities [Member]
Class Of Stock [Line Items]
Stock repurchases and redeemed or repurchases authorized amount						$ 200,000,000.0
Class B Common Stock [Member]
Class Of Stock [Line Items]
Common stock voting rights description			Each share of Class B common stock is entitled to 10 votes
Common stock, shares authorized	500,000,000		500,000,000	500,000,000
Common stock, par value	$ 0.01		$ 0.01	$ 0.01
Conversion of common stock			1
Common stock conversion features			Each share of Class B common stock is entitled to 10 votes and is convertible at any time into one share of Class A common stock.
Common stock, shares outstanding	21,285,533		21,285,533	21,285,533