Investments - Schedule of Summarized Financial Information, Equity Fund (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Investments in Unconsolidated Affiliates
Cash and cash equivalents	$ 444.3	$ 533.7
Total assets	2,705.1	2,092.2	$ 1,957.8
Total liabilities	500.1	562.4
Equity Fund | Unconsolidated affiliates
Investments in Unconsolidated Affiliates
Due from brokers and counterparties	393.5	93.2
Cash and cash equivalents	0.0	45.2
Derivative contracts, at fair value	0.0	2.4
Other assets	0.1	0.0
Total assets	393.6	140.8
Derivative contracts, at fair value	118.1	0.0
Capital received in advance	0.0	45.2
Other liabilities	1.4	0.2
Total liabilities	119.5	45.4
Net assets	274.1	$ 95.4
Equity in Losses of Unconsolidated Affiliates
Total net investment loss	(0.3)
Realized gain on securities and derivative contracts	0.5
Change in unrealized loss on derivative contracts	(120.5)
Change in net assets from operations	$ (120.3)