Fair Value of Financial Instruments - Fair Value Assets Measured on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Beneficial interests in securitizations	$ 19,531
Money market funds
Assets:
Money market funds	19,810	$ 63,713
Level 1
Assets:
Beneficial interests in securitizations	0
Level 1 | Money market funds
Assets:
Money market funds	19,810	63,713
Level 2
Assets:
Beneficial interests in securitizations	19,531
Level 2 | Money market funds
Assets:
Money market funds	0	0
Level 3
Assets:
Beneficial interests in securitizations	0
Level 3 | Money market funds
Assets:
Money market funds	$ 0	$ 0