Summary of Significant Accounting Policies - Investments in Unconsolidated Variable Interest Entities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Investments	$ 1,302.3	$ 1,107.0
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Investments	771.1	664.2
Due from affiliates, net	0.0	1.8
Maximum Exposure to Loss	771.1	$ 666.0
Amount of variable interests in unconsolidated VIEs related to performance fees	44.7
Amount of variable interests in unconsolidated VIEs related to management fees	$ 10.6