Business Changes and Developments - Additional Information (Detail) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended				1 Months Ended					12 Months Ended					1 Months Ended	12 Months Ended							3 Months Ended	12 Months Ended		1 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 29, 2011 Person	Aug. 19, 2011 Class A USD ($)	Dec. 31, 2012 Class A USD ($)	Dec. 31, 2011 Class A USD ($)	Dec. 31, 2011 Deferred Payments USD ($)	Dec. 31, 2011 Deferred Payments GBP (£)	Dec. 31, 2011 Deferred Payments Class A	Dec. 31, 2011 Lexicon Partnership LLP USD ($)	Dec. 31, 2011 Lexicon Partnership LLP GBP (£)	Aug. 19, 2011 Lexicon Partnership LLP USD ($)	Aug. 19, 2011 Lexicon Partnership LLP GBP (£)	Aug. 19, 2011 Lexicon Partnership LLP Class A	Dec. 31, 2012 Lexicon Partnership LLP Client Related Intangible Assets USD ($)	Dec. 31, 2011 Lexicon Partnership LLP Client Related Intangible Assets USD ($)	Dec. 31, 2012 ABS	Dec. 29, 2011 ABS USD ($) Person	Dec. 29, 2011 Parent Company Person	Dec. 31, 2012 Parent Company Class A USD ($)	Dec. 31, 2011 Parent Company Class A USD ($)	Sep. 30, 2011 EAM USD ($)	Dec. 31, 2011 EAM USD ($)	Dec. 31, 2010 EAM USD ($)	Dec. 31, 2012 Mt Eden Investment Advisors Llc USD ($)	Dec. 31, 2012 Mt Eden Investment Advisors Llc Minimum	Dec. 31, 2012 Mt Eden Investment Advisors Llc Maximum	Dec. 31, 2012 Mt Eden Investment Advisors Llc Noncontrolling Interest USD ($)	Dec. 31, 2012 Mt Eden Investment Advisors Llc Client Related Intangible Assets USD ($)	Dec. 31, 2012 Mt Eden Investment Advisors Llc Non-compete agreements USD ($)	Dec. 31, 2012 Mt Eden Investment Advisors Llc Other Intangible Assets USD ($)
Business Acquisition [Line Items]
Cash received by sellers for consideration											$ 9,274	£ 5,619	$ 52,160	£ 31,598					$ 45,104							$ 6,917
Value of equity interest paid to sellers for consideration													636																2,694
Approximate amount to be received by sellers of acquiree entity								14,733	8,925				76,167	46,142												282
Funded repayment of outstanding capital notes											8,318	5,039														1,047
Business acquisition purchase price allocation, goodwill													44,286													6,500
Business acquisition purchase price allocation, intangible assets													7,164				7,164													3,630	169	445
Amortization period of intangible assets																	6 months										2 years	10 years
Approximate number of shares of common stock receivable by sellers of acquiree entity					28										1,911
Common Stock, par value					$ 0.01	$ 0.01	$ 0.01														$ 0.01	$ 0.01
Shares of common stock to be issued for purchase consideration										1,883
Period of deferral of issue of class of restricted shares, years		4 years
Gross fair value of accounts receivable													7,821
Amortization expense of client related intangible assets recognized	10,872	14,315	7,541								5,075					2,089	5,075
Revenue related to acquiree from the period of acquisition											41,637
Operating expenses related to acquiree from the period of acquisition											32,073
Income resulting out of acquisition from the period of acquisition											9,564
Other expenses incurred related to acquisition											23,587
Compensation charges											14,618
Acquisition related special charges included in other expenses related to acquisition											3,894
Percentage of non-controlling interest ownership																		45.00%	45.00%
Remaining interest																			55.00%
Number of governing management committee members				5															3	2
Charge in the investment management segment related to the write-off of client-based intangible assets																							975
Pretax income or losses		$ (4,198)	$ (2,618)																					$ (4,198)	$ (2,618)