Operating Expenses - Components of Operating Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Income and Expenses [Abstract]
Fixed compensation expense	$ 9,286	$ 6,872	$ 18,441	$ 13,664
Variable compensation expense	3,578	3,243	7,569	5,974
Equity compensation expense	3,539	2,824	6,277	5,154
Total compensation expense	16,403	12,939	32,287	24,792
Systems and consulting	2,242	3,977	4,364	7,443
Accounting and legal	1,130	1,183	2,707	2,816
Office costs	1,366	1,170	2,598	2,155
Corporate costs	512	558	1,037	1,111
Other operating expenses	3,565	2,455	7,288	3,937
Total Operating Expenses	$ 25,218	$ 22,282	$ 50,281	$ 42,254