Variable Interest Entities - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Consolidated investment entities
Variable Interest Entity [Line Items]
Investments		$ 1,957.9
Recurring
Variable Interest Entity [Line Items]
Total financial assets	$ 17,493.9	16,406.8
Financial liabilities	2,039.0	1,978.3
Recurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets		1,990.8
Financial liabilities		1,603.1
Recurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial liabilities		1,603.1
Recurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments		32.9
Recurring | Consolidated investment entities | Corporate debt securities
Variable Interest Entity [Line Items]
Investments		1,850.7
Recurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments		107.2
Level 1 | Recurring
Variable Interest Entity [Line Items]
Total financial assets	3,669.9	3,311.8
Financial liabilities	1,868.5	1,694.0
Level 1 | Recurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets		32.9
Financial liabilities		0.0
Level 1 | Recurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial liabilities		0.0
Level 1 | Recurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments		32.9
Level 1 | Recurring | Consolidated investment entities | Corporate debt securities
Variable Interest Entity [Line Items]
Investments		0.0
Level 1 | Recurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments		0.0
Level 2 | Recurring
Variable Interest Entity [Line Items]
Total financial assets	13,737.0	13,031.8
Financial liabilities	170.5	284.1
Level 2 | Recurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets		1,850.7
Financial liabilities		1,603.1
Level 2 | Recurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial liabilities		1,603.1
Level 2 | Recurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments		0.0
Level 2 | Recurring | Consolidated investment entities | Corporate debt securities
Variable Interest Entity [Line Items]
Investments		1,850.7
Level 2 | Recurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments		0.0
Level 3 | Recurring
Variable Interest Entity [Line Items]
Total financial assets	87.0	63.2
Financial liabilities	$ 0.0	0.2
Level 3 | Recurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets		107.2
Financial liabilities		0.0
Level 3 | Recurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial liabilities		0.0
Level 3 | Recurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments		0.0
Level 3 | Recurring | Consolidated investment entities | Corporate debt securities
Variable Interest Entity [Line Items]
Investments		0.0
Level 3 | Recurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments		$ 107.2