Summary of Significant Accounting Policies (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) Segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Fees paid to unaffiliated advisers	$ 47.2	$ 76.4	$ 124.4
Comprehensive income (loss), net of tax, equity investments	$ 0.2	$ (0.4)
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 and equipment	3 years
Furniture and office equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	7 years
Computer equipment and software [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	3 years
Computer equipment and software [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	5 years