Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012
Summary Of Significant Accounting Policies [Line Items]
Investment management fees	$ 1.6	$ 1.7	$ 2.5
Number of operating segment	1
Management fees net of fees paid	$ 124.4	$ 96.1	$ 53.7
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred sales charges receiving period	1 year
Deferred commissions amortization period	1 year
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]
Deferred sales charges receiving period	5 years
Deferred commissions amortization period	5 years
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	7 years
Maximum [Member] | Computer Equipment and Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Useful lives of fixed assets	5 years