Fair Value Disclosures - Narrative (Details)	9 Months Ended
	Sep. 30, 2017
Non-agency MBS | Re-REMIC Trusts | 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	18.50%
Private Equity Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average remaining life of private equity and similar investments	3 years
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	12
Private Equity Investments | Weighted average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unobservable inputs, EBITDA multiples	8