Fair Value Disclosures - Narrative (Details)	12 Months Ended
	Dec. 31, 2017
Non-agency MBS | Re-REMIC | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Difference between fair value of Re-REMIC non-agency MBS held and the fair value of the underlying securities, percentage	21.10%
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	14
Private Equity Investments | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	9