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Loans - Investment of Originated Loans Held-for-Investment by Loan Type and Credit Quality Indicator (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	$ 3,213,689	$ 2,987,067
Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	3,197,089	2,968,346
Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	2,531	2,850
Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	14,069	15,871
Real Estate | Multifamily | Less Than 35% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	248,705	233,251
Real Estate | Multifamily | Less Than 35% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	248,120	232,950
Real Estate | Multifamily | Less Than 35% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Real Estate | Multifamily | Less Than 35% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	585	301
Real Estate | Multifamily | Equal to or Greater than 35% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	2,055,461	1,966,483
Real Estate | Multifamily | Equal to or Greater than 35% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	2,049,444	1,960,984
Real Estate | Multifamily | Equal to or Greater than 35% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	760	296
Real Estate | Multifamily | Equal to or Greater than 35% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	5,257	5,203
Real Estate | Commercial | Less Than 35% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	90,805	79,855
Real Estate | Commercial | Less Than 35% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	90,805	79,485
Real Estate | Commercial | Less Than 35% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	370
Real Estate | Commercial | Less Than 35% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Real Estate | Commercial | Equal to or Greater than 35% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	445,440	449,432
Real Estate | Commercial | Equal to or Greater than 35% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	438,479	440,065
Real Estate | Commercial | Equal to or Greater than 35% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	808	1,092
Real Estate | Commercial | Equal to or Greater than 35% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	6,153	8,275
Real Estate | One-to-Four Family | Less Than 60% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	55,838	55,060
Real Estate | One-to-Four Family | Less Than 60% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	53,521	52,886
Real Estate | One-to-Four Family | Less Than 60% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	590	777
Real Estate | One-to-Four Family | Less Than 60% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	1,727	1,397
Real Estate | One-to-Four Family | Equal to or Greater than 60% LTV
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	28,351	30,295
Real Estate | One-to-Four Family | Equal to or Greater than 60% LTV | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	28,326	29,967
Real Estate | One-to-Four Family | Equal to or Greater than 60% LTV | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Real Estate | One-to-Four Family | Equal to or Greater than 60% LTV | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	25	328
Real Estate | Construction and Land
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	46,843	38,303
Real Estate | Construction and Land | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	46,843	38,303
Real Estate | Construction and Land | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Real Estate | Construction and Land | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Real Estate | Home Equity and Lines of Credit
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	90,110	86,848
Real Estate | Home Equity and Lines of Credit | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	89,749	86,547
Real Estate | Home Equity and Lines of Credit | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	76	14
Real Estate | Home Equity and Lines of Credit | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	285	287
Commercial and Industrial
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	149,725	45,456
Commercial and Industrial | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	149,391	45,075
Commercial and Industrial | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	297	301
Commercial and Industrial | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	37	80
Other
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	2,411	2,084
Other | Pass
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	2,411	2,084
Other | Special Mention
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	0	0
Other | Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
Originated loans held-for-investment, net	$ 0	$ 0

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