Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Oct. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary Of Significant Accounting Policies [Line Items]
Marketable Securities Realized Gain Loss Description			Less than $(0.1) million
Company recognized realized gains		$ 400,000		$ 300,000
Estimated useful lives of intangibles	5 years
Repurchased common shares		245,000	155,000
No of Repurchased common shares		17,734,000	8,794,000
Number of shares available				350,000
Aggregate assets		700,000,000	1,000,000,000	1,000,000,000
Investment management fee		2,500,000	3,300,000	4,600,000
management fees net of fees paid		53,700,000	36,400,000	24,000,000
Advertising and promotional expense		$ 500,000	$ 300,000	$ 200,000
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles		1 year
Minimum [Member] | Furniture and office equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of fixed assets		3 years
Minimum [Member] | Computer equipment and software [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of fixed assets		3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of intangibles		16 years
Maximum [Member] | Furniture and office equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of fixed assets		10 years
Maximum [Member] | Computer equipment and software [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of fixed assets		5 years