Summary of Significant Accounting Policies (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of operating segments | Segment	1
Fees paid to unaffiliated advisers	$ 46,700	$ 47,200	$ 76,400
Cumulative effect adjustment for adoption of ASU 2016-09		1,051
Comprehensive income (loss), net of tax, equity investments	$ (400)	200
Minimum
Summary Of Significant Accounting Policies [Line Items]
Deferred commissions amortization period	1 year
Deferred sales charges amortization period	1 year
Weighted Average of Useful Life	1 year
Maximum
Summary Of Significant Accounting Policies [Line Items]
Deferred commissions amortization period	5 years
Deferred sales charges amortization period	5 years
Weighted Average of Useful Life	16 years
Furniture and office equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	3 years
Furniture and office equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	7 years
Computer equipment and software | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	3 years
Computer equipment and software | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of furniture and equipment	5 years
Accounting Standards Update 2016-09
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment for adoption of ASU 2016-09		$ 1,100