Commitments, Contingencies and Guarantees (Commitments to Fund Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Limited Partner [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	$ 579,647	$ 523,903
Commercial Loan [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	147,898	22,605
Bank Loans [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	77,854	137,076
Equity Release Mortgages [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Liability	$ 215,305	$ 264,858