Principles of Consolidation - Summary of Unconsolidated Variable Interest Entity's Sponsored by Redwood (Details) - Variable Interest Entity, Not Primary Beneficiary - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
On-balance sheet assets, at fair value:
Maximum loss exposure	$ 381,661	$ 334,734
Principal balance of loans outstanding	8,364,148	6,870,398
Principal balance of loans 30 days delinquent	27,926	21,427
Interest-only, senior and subordinate securities, classified as trading
On-balance sheet assets, at fair value:
Securities	101,426	41,909
Subordinate securities, classified as AFS
On-balance sheet assets, at fair value:
Securities	219,255	234,025
Mortgage servicing rights
On-balance sheet assets, at fair value:
Securities	$ 60,980	$ 58,800