Significant Accounting Policies - Basis of Presentation (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Dec. 21, 2017	Nov. 09, 2017	Jul. 11, 2016
Variable Interest Entity [Line Items]
Investments in firm-sponsored vehicles where we are not primary beneficiary	$ 2.9	$ 3.0
Maximum loss exposure	3.1	3.2
Pzena International Value Service, a series of Pzena Investment Management International, LLC
Variable Interest Entity [Line Items]
Equity method investment, assets	3.4
VIEs that are not consolidated
Variable Interest Entity [Line Items]
Equity method investment, assets	$ 169.9	$ 165.5
Pzena Small Cap Focused Value Fund, a series of Advisors Series Trust
Variable Interest Entity [Line Items]
Operating company's ownership at end of period					36.10%
Pzena Mid Cap Focused Value Fund, a series of Advisors Series Trust
Variable Interest Entity [Line Items]
Operating company's ownership at end of period				41.70%
Pzena Long/Short Value Fund, a series of Advisors Series Trust
Variable Interest Entity [Line Items]
Operating company's ownership at end of period			35.50%