Fair Value of Financial Instruments - Losses Incurred for Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Residential mortgage loans held in securitization trusts – impaired loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gains (losses) for assets measured at fair value on a non-recurring basis	$ (472)	$ (482)	$ (1,261)
Real estate owned held in residential securitization trusts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gains (losses) for assets measured at fair value on a non-recurring basis	$ (6)	$ (130)	$ 100

This item represents the estimated amount of gains or losses included in earnings during the period attributable to changes in instrument-specific credit risk arising from loans and other receivables held as assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

Name:	us-gaap_FairValueOptionCreditRiskGainsLossesOnAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration