DEBT OBLIGATIONS AND CREDIT FACILITIES - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Debt obligations	$ 585,714,000	$ 615,179,000
2.50% Debt, Term Loan Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	250,000,000
Debt obligations	225,000,000	243,750,000
Debt Instrument, Loan Amortization Rate (as a percent)	2.50%
Old Line of Credit Facility [Member]
Debt Instrument [Line Items]
Debt obligations		247,500,000.0
Senior Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Minimum required levels of assets under management	50,000,000,000
Minimum required levels of net worth	600,000,000
Senior Unsecured Credit Facility [Member] | 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 (as a percent)	2.60%
Senior Unsecured Credit Facilities [Member] | Senior Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
LIBOR margin (as a percent)	1.00%
Commitment fee payable on unused funds (as a percent)	0.125%
Senior Unsecured Credit Facilities [Member] | Revolving credit facility [Member]
Debt Instrument [Line Items]
Credit agreement	$ 500,000,000
Senior Unsecured Credit Facilities [Member] | Term Loan and Revolver [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Term (in years)	5 years
Numerator [Member] | Senior Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Maximum leverage ratio	3.0
Minimum fixed charge coverage ratio	2.5
Denominator [Member] | Senior Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Maximum leverage ratio	1.0
Minimum fixed charge coverage ratio	1.0