Investments (excluding Consolidated Investment Entities) - Impaired Loans (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Impaired loans without allowances for losses	$ 4,000,000	$ 5,000,000
Less: Allowances for losses on impaired loans	0	0
Impaired loans, net	4,000,000	5,000,000
Unpaid principal balance of impaired loans	6,000,000	6,000,000
Mortgage Loans in Process of Foreclosure, Amount	0	0
Loans 30 days or less in arrears
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in arrears, amortized cost	$ 0.0	$ 0