Principles of Consolidation - Summary of Unconsolidated Variable Interest Entity's Sponsored by Redwood (Details) - Variable Interest Entity, Not Primary Beneficiary - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
On-balance sheet assets, at fair value:
Maximum loss exposure	$ 356,793	$ 381,661
Assets transferred:
Principal balance of loans outstanding	10,349,405	8,364,148
Principal balance of loans 30 days delinquent	19,625	27,926
Interest-only, senior and subordinate securities, classified as trading
On-balance sheet assets, at fair value:
Securities	130,598	101,426
Subordinate securities, classified as AFS
On-balance sheet assets, at fair value:
Securities	163,870	219,255
Mortgage servicing rights
On-balance sheet assets, at fair value:
Securities	$ 62,325	$ 60,980