Loans Payable - Additional Information (Detail) (USD $)	1 Months Ended			12 Months Ended				1 Months Ended				1 Months Ended	12 Months Ended			12 Months Ended					12 Months Ended			12 Months Ended	1 Months Ended						12 Months Ended				12 Months Ended
	Mar. 31, 2012	Oct. 31, 2011	Mar. 31, 2008	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	May 09, 2012	Dec. 31, 2010 Mubadala [Member]	Dec. 31, 2012 Mubadala [Member]	Mar. 31, 2008 Interest Rate Swap [Member]	Mar. 31, 2008 Interest Rate Swap One [Member]	Dec. 31, 2011 Interest Rate Swap 2 [Member]	Dec. 31, 2012 Term Loan [Member]	Dec. 31, 2011 Term Loan [Member]	Dec. 31, 2012 Senior Credit Facility [Member]	Dec. 31, 2012 Claren Road [Member]	Dec. 31, 2012 Subordinated Notes [Member]	Dec. 31, 2011 Subordinated Notes [Member]	Dec. 31, 2010 Subordinated Notes [Member]	Dec. 31, 2012 Subordinated Notes [Member] Payment-in-kind notes [Member]	Dec. 31, 2012 Maximum [Member] Ratio	Dec. 31, 2012 Minimum [Member] Ratio	Dec. 31, 2012 Loan Agreement with a Financial Institution [Member]	Dec. 31, 2012 Subsequent Event [Member]	Mar. 31, 2008 Subsequent Event [Member] Interest Rate Swap One [Member]	Mar. 31, 2008 Subsequent Event [Member] Interest Rate Swap 2 [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	May 08, 2012 Revolving Credit Facility [Member]	Dec. 31, 2011 Revolving Credit Facility [Member]	Dec. 31, 2012 Liquidity Facility Agreements [Member]	Dec. 31, 2012 New Senior Credit Facility [Member]	Dec. 31, 2011 New Senior Credit Facility [Member]	Dec. 31, 2010 New Senior Credit Facility [Member]	Sep. 09, 2012 New Senior Credit Facility [Member]	Dec. 31, 2012 3.875% Senior Notes [Member]
Line of Credit Facility [Line Items]
Maximum Borrowing capacity in term loan							$ 500,000,000			$ 725,000,000				$ 500,000,000
Borrowing capacity							750,000,000																						750,000,000	40,000,000
Loan Maturity				Feb. 01, 2023									Sep. 30, 2016											Dec. 31, 2015	Aug. 20, 2013	Sep. 30, 2016									Feb. 01, 2023
Maximum Percentage of applicable margin in addition to base rate																																		0.75%
Maximum percentage of applicable margin																															1.25%			1.75%
Outstanding amounts of term loan payable				14,577,800,000	10,718,400,000									500,000,000																	500,000,000
Outstanding borrowing under credit facility																											386,300,000	618,100,000	310,900,000
Minimum Management fee earning assets																															61,800,000,000
Minimum percentage required to maintain for future acquired assets under management																															70.00%
Leverage Ratio																					3.0	1.0
Interest expense																															20,400,000	20,900,000	17,300,000
Percentage of borrowings under interest rate risk derivative			33.00%
Weighted Average Interest Rate											3.319%
Notional amount											239,200,000
Percentage of interest on term loan borrowings												1.082%
Unrealized losses for the period				(10,200,000)	(4,300,000)	(7,300,000)
Accumulated other comprehensive loss related to cash flow hedges				5,000,000
Senior notes principal amount				500,000,000																															500,000,000
Interest rate on senior notes				3.875%																															3.875%
Senior notes percentage of par value				99.966%																															99.966%
Senior notes redemption terms percentage to principal amount																																			100.00%
Senior notes interest rate description																																			Treasury rate plus 30 basis points plus accrued and unpaid interest on the principal amounts
Frequency of repayment of debt																																			Semi-annually on February 1 and August 1
Date of first required interest payment																																			Aug. 01, 2013
Repayment of debt															75,000,000
Loan Agreement				47,500,000
Annual interest rate on loan amount adjustable				6.00%
Loan agreement															500,000,000								50,000,000
Business Acquisition Loan Acquired Effective Percentage																8.00%
Business acquisition loan maturity date																Jan. 03, 2017
Business acquisition loan acquired principal balance remaining to be repaid																10,000,000
Proceeds from Issuance of Subordinated Long-term Debt								494,000,000
Subordinated notes									500,000,000
Percentage of equity interest given to affiliate				2.00%
Debt instrument interest rate payable in cash																	7.25%		7.25%
Debt instrument interest rate payable by issue of payment in kind notes																				7.50%
Borrowings under credit facility		265,500,000
Aggregate principal amount redeem	250,000,000	250,000,000
Redemption value of subordinated debt to be redeemed in cash	260,000,000	260,000,000
Percentage of premium paid on redemption of subordinated notes	4.00%	4.00%
Accrued interest	3,100,000	5,500,000
Borrowings under credit facility	263,100,000			820,000,000	520,500,000
Interest expense				24,600,000	60,600,000	17,800,000											3,100,000	33,600,000
Fair value of collateralized loan obligation assets				15,700,000,000	11,000,000,000
Long-term debt, weighted average interest rate																														0.25%
Maximum borrowing																														$ 13,200,000