Stock Transactions and Unit Redemptions - Unit Redemption and Share Repurchase Activity (Parenthetical) (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Schedule Of Share Repurchase Programs [Line Items]
Average price for redemption of limited partnership units	$ 5.54	$ 6.47	$ 4.71	$ 6.61
Repurchase of Class A common stock, shares			33,478
Aggregate purchase price of Class A common stock			$ 188	$ 337
Average price of Class A common stock			$ 5.61
Class A Common Stock [Member]
Schedule Of Share Repurchase Programs [Line Items]
Average price for redemption of founding/working partner units	$ 4.68	$ 7.76	$ 3.78	$ 6.55
Repurchase of Class A common stock, shares	33,478		33,478	44,013	44,013
Aggregate purchase price of Class A common stock	$ 200		$ 200	$ 300
Average price of Class A common stock	$ 5.61		$ 5.61	$ 7.66
Limited Partnership [Member]
Schedule Of Share Repurchase Programs [Line Items]
Redeemed limited partnership units	2,500,000	2,600,000	6,900,000	5,400,000
Other Ownership Interest [Member]
Schedule Of Share Repurchase Programs [Line Items]
Redeemed limited partnership units	200,000	300,000	900,000	1,300,000