Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended			6 Months Ended		12 Months Ended
	Feb. 24, 2016	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Average price for redemption of limited partnership units					$ 11.20		$ 8.93	$ 10.97	$ 8.85
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired		27,634	121,023	248,882		552,955		950,494
Aggregate purchase price of Class A common stock								$ 10,590	$ 69,730
Average price of Class A common stock		$ 12.21	$ 11.57	$ 10.99		$ 11.07		$ 11.14
Class A Common Stock [Member]
Class of Stock [Line Items]
Repurchase of Class A common stock, shares Treasury Stock Shares Acquired					397,539		797,189	950,494	7,984,235	10,823,942
Aggregate purchase price of Class A common stock					$ 4,500		$ 7,200	$ 10,600	$ 69,700
Average price of Class A common stock	$ 8.40				$ 11.25		$ 9.04	$ 11.14	$ 8.75
Limited Partnership [Member]
Class of Stock [Line Items]
Redeemed limited partnership units					1,600,000		1,800,000	4,600,000	2,400,000
Aggregate redemption price of limited partnership units					$ 18,500		$ 15,800	$ 50,000	$ 21,600
Other Ownership Interest [Member]
Class of Stock [Line Items]
Redeemed limited partnership units					300,000		39,600	300,000	100,000
Aggregate redemption price of limited partnership units					$ 2,800		$ 300	$ 3,000	$ 1,100
Average price for redemption of founding/working partner units					$ 10.72		$ 8.01	$ 10.72	$ 8.17