Components of Operating Expenses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Component of Operating Other Cost and Expense [Abstract]
Fixed compensation expense	$ 6,067	$ 4,373	$ 11,690	$ 9,208
Variable compensation expense	3,960	3,024	8,797	5,617
Equity compensation expense	3,396	2,958	5,883	5,134
Severance expense	3,366		3,434	200
Total compensation expense	16,789	10,355	29,804	20,159
Accounting and legal	805	1,048	3,052	2,166
Systems and consulting	2,318	1,303	4,060	2,484
Office costs	1,355	1,682	2,652	3,423
Other operating expenses	2,377	777	3,673	1,567
Total Operating Expenses	$ 23,644	$ 15,165	$ 43,241	$ 29,799