Summary of Significant Accounting Policies - Loans Purchased and Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Loans Purchased
Purchase of loans receivable held for investment	$ 0	$ 24,640
Allowance for Loan Losses
Length of period over which historical loss experience	5 years
Single family
Loans Purchased
Purchase of loans receivable held for investment		$ 24,600

The cash outflow from (a) purchases of loans held-for-investment, (b) purchases of leases held-for-investment, and (c) both.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585

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