Consolidated Investment Products (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) voting_interest_entity	Dec. 31, 2017 USD ($) variable_interest_entity	Dec. 31, 2016 USD ($) variable_interest_entity
Variable Interest Entity [Line Items]
Variable Interest Entity Unavailability of Information Number of Entities No Longer Consolidated				1	8	10
Deconsolidation, Gain (Loss), Amount			$ 0			$ 0
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	$ 227,300,000	$ 227,300,000	227,300,000	$ 227,300,000	$ 227,300,000	$ 234,400,000
Collateralized Loan Obligations Terms Of Arrangements Interest Rate Margin Spread High		10.00%
Collateral Assets Default Percentage		0.41%				0.30%
Notes Issued By Collateralized Loan Obligations Terms Of Arrangements Interest Rate Margin Spread Low		0.92%
Notes Issued By Collateralized Loan Obligations Terms Of Arrangements Interest Rate Margin Spread High		8.25%
Variable Interest Entity Consolidated Number of Entities | variable_interest_entity					19
CLO [Member]
Variable Interest Entity [Line Items]
Issued Note Maturity Average Years	10 years
Senior Secured Bank Loans And Bonds [Member] | CLO [Member]
Variable Interest Entity [Line Items]
Fair Value, Option, Aggregate Differences, Long-term Debt Instruments	$ 84,600,000	$ 84,600,000	84,600,000	84,600,000	$ 84,600,000	$ 96,600,000
Minimum [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity Term	7 years
Maximum [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity Term	12 years
Consolidation, Eliminations [Member] | Bank Loan Obligations [Member] | CLO [Member]
Variable Interest Entity [Line Items]
Collateralized Loan Obligation, Collateral Assets	$ 4,859,300,000	$ 4,859,300,000	$ 4,859,300,000	$ 4,859,300,000	$ 4,859,300,000	$ 4,397,800,000