Summary of Significant Accounting Policies (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Segment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Investment of capital	$ 40.0
Investment management fees	$ 0.9	$ 1.6	$ 1.7
Number of operating segments | Segment	1
Fees paid to unaffiliated advisers	$ 76.4	$ 124.4	$ 96.1
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred commissions amortization period	1 year
Deferred sales charges amortization period	1 year
Estimated useful lives of intangibles	1 year
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Deferred commissions amortization period	5 years
Deferred sales charges amortization period	5 years
Estimated useful lives of intangibles	16 years
Furniture and office equipment [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture, equipment and leasehold improvements, net	3 years
Furniture and office equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture, equipment and leasehold improvements, net	7 years
Computer equipment and software [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture, equipment and leasehold improvements, net	3 years
Computer equipment and software [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture, equipment and leasehold improvements, net	5 years