Fair Value Disclosures - Narrative (Details)	Dec. 31, 2018 Multiple
Private Equity Investments | Minimum | EBITDA Multiple
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	6
Private Equity Investments | Maximum | EBITDA Multiple
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	14
Private Equity Investments | Weighted average | EBITDA Multiple
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	9
Re-REMIC | Non-agency MBS | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Difference between fair value of securities held and the fair value of the underlying securities, percentage	18.60%