Fair Value Disclosures - Narrative (Details)	3 Months Ended
	Mar. 31, 2018
Private Equity Investments | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	5
Private Equity Investments | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	15
Private Equity Investments | Weighted average
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 vaue of the underlying securities, percentage	16.30%