Variable Interest Entities - Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Financial Liabilities	$ 1,929.3	$ 1,770.1
Nonrecurring
Variable Interest Entity [Line Items]
Investments:	84.4	74.8
Total financial assets	763.6	709.7
Nonrecurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets	709.5
Financial Liabilities	450.7
Nonrecurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial Liabilities	450.7
Nonrecurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments:	54.5
Nonrecurring | Consolidated investment entities | Corporate Debt Securities
Variable Interest Entity [Line Items]
Investments:	570.3
Nonrecurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments:	84.7
Level 1
Variable Interest Entity [Line Items]
Financial Liabilities	1,706.4	1,537.8
Level 1 | Nonrecurring
Variable Interest Entity [Line Items]
Investments:	36.3	38.5
Total financial assets	36.3	38.5
Level 1 | Nonrecurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets	54.5
Financial Liabilities	0.0
Level 1 | Nonrecurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial Liabilities	0.0
Level 1 | Nonrecurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments:	54.5
Level 1 | Nonrecurring | Consolidated investment entities | Corporate Debt Securities
Variable Interest Entity [Line Items]
Investments:	0.0
Level 1 | Nonrecurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments:	0.0
Level 2
Variable Interest Entity [Line Items]
Financial Liabilities	166.4	178.2
Level 2 | Nonrecurring
Variable Interest Entity [Line Items]
Investments:	0.0	0.0
Total financial assets	0.0	0.0
Level 2 | Nonrecurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets	570.3
Financial Liabilities	450.7
Level 2 | Nonrecurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial Liabilities	450.7
Level 2 | Nonrecurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments:	0.0
Level 2 | Nonrecurring | Consolidated investment entities | Corporate Debt Securities
Variable Interest Entity [Line Items]
Investments:	570.3
Level 2 | Nonrecurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments:	0.0
Level 3
Variable Interest Entity [Line Items]
Financial Liabilities	56.5	54.1
Level 3 | Nonrecurring
Variable Interest Entity [Line Items]
Investments:	48.1	36.3
Total financial assets	727.3	$ 671.2
Level 3 | Nonrecurring | Consolidated investment entities
Variable Interest Entity [Line Items]
Total financial assets	84.7
Financial Liabilities	0.0
Level 3 | Nonrecurring | Consolidated investment entities | Collateralized loan obligation notes
Variable Interest Entity [Line Items]
Financial Liabilities	0.0
Level 3 | Nonrecurring | Consolidated investment entities | Cash and cash equivalents
Variable Interest Entity [Line Items]
Investments:	0.0
Level 3 | Nonrecurring | Consolidated investment entities | Corporate Debt Securities
Variable Interest Entity [Line Items]
Investments:	0.0
Level 3 | Nonrecurring | Consolidated investment entities | Real estate fund
Variable Interest Entity [Line Items]
Investments:	$ 84.7