Fair Value of Assets and Liabilities - Sensitivity of Fair Value of Loans Invested in by Company (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans Invested in by Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans Receivable and Loans Held for Sale, Fair Value Disclosure	$ 597,055	$ 25,911
Fair Value Assumptions, Expected Term	1 year 6 months
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Discount Rate	$ (7,449)
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Credit Loss Rates on Underlying Loans	(10,090)
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Expected Prepayment Rates	(3,548)
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Discount Rate	(14,715)
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Credit Loss Rates on Underlying Loans	(18,935)
Sensitivity Analysis of Fair Value, Loans Invested in by Company, Impact of Adverse Change in Expected Prepayment Rates	$ (5,894)