Consolidated Investment Entities (Fair Value Hierarchy) (Details) (Measured at fair value on a recurring basis, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	$ 9,443.1	$ 8,894.6
Liabilities	5,525.5	5,161.6
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	570.0	710.7
Liabilities	0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	6,095.6	5,424.3
Liabilities	0	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	2,777.5	2,759.6
Liabilities	5,525.5	5,161.6
VOEs | Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	71.4	68.2
VOEs | Cash and Cash Equivalents | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	71.4	68.2
VOEs | Cash and Cash Equivalents | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VOEs | Cash and Cash Equivalents | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VOEs | Limited Partnerships
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	3,382.4	3,218.6
VOEs | Limited Partnerships | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VOEs | Limited Partnerships | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	629.9	484.5
VOEs | Limited Partnerships | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	2,752.5	2,734.1
VIEs | Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	498.6	642.5
VIEs | Cash and Cash Equivalents | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	498.6	642.5
VIEs | Cash and Cash Equivalents | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VIEs | Cash and Cash Equivalents | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VIEs | Corporate Loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	5,490.7	4,965.3
VIEs | Corporate Loans | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	0	0
VIEs | Corporate Loans | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	5,465.7	4,939.8
VIEs | Corporate Loans | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	25.0	25.5
VIEs | Collateralized Debt Obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	5,525.5	5,161.6
VIEs | Collateralized Debt Obligations | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0	0
VIEs | Collateralized Debt Obligations | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0	0
VIEs | Collateralized Debt Obligations | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	$ 5,525.5	$ 5,161.6