DEBT OBLIGATIONS AND CREDIT FACILITIES - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Minimum required levels of assets under management	$ 50,000,000,000
Minimum required levels of assets under net worth	600,000,000
Debt obligations	608,929,000		615,179,000
2.50% Debt, Term Loan Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	250,000,000
Fixed interest rate as a result of interest rate swap	2.60%
Final principal payment	125,000,000
Debt obligations	237,500,000		243,750,000
Debt Instrument, Loan Amortization Rate	2.50%
Old Line of Credit Facility [Member]
Debt Instrument [Line Items]
Debt obligations	247,500,000
Revolving credit facility [Member]
Debt Instrument [Line Items]
Credit agreement	150,000,000
Eurodollar margin	1.75%	[1]
Maturity	Dec. 15, 2013
Senior Unsecured Credit Facilities [Member] | Revolving credit facility [Member]
Debt Instrument [Line Items]
Credit agreement	$ 500,000,000
Eurodollar margin	1.00%
Commitment fee payable on unused funds	0.125%
Numerator [Member]
Debt Instrument [Line Items]
Maximum leverage ratio	3.0
Minimum fixed charge coverage ratio	2.5
Denominator [Member]
Debt Instrument [Line Items]
Maximum leverage ratio	1.0
Minimum fixed charge coverage ratio	1.0

[1] The facilities bear interest, at the borrower's option, at (a) an annual rate of LIBOR plus the applicable margin or (b) an alternate base rate, as defined in the respective credit agreement.