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Debt Securities Available-for-Sale - Comparative Summary of Mortgage-Backed Securities and Other Securities Available-for-Sale) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,083,047	$ 1,246,673
Gross unrealized gains	15,315	18,862
Gross unrealized losses	(939)	(730)
Estimated fair value	1,097,423	1,264,805
U.S. Government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	2,758	3,168
Gross unrealized gains	0	0
Gross unrealized losses	(4)	(10)
Estimated fair value	2,754	3,158
Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	956,534	1,155,285
Gross unrealized gains	14,499	17,747
Gross unrealized losses	(888)	(720)
Estimated fair value	970,145	1,172,312
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	123,755	88,220
Gross unrealized gains	816	1,115
Gross unrealized losses	(47)	0
Estimated fair value	124,524	89,335
GSE | Pass-through Certificates
Debt Securities, Available-for-sale [Line Items]
Amortized cost	221,513	270,867
Gross unrealized gains	8,640	10,720
Gross unrealized losses	(386)	(244)
Estimated fair value	229,767	281,343
GSE | Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	735,021	884,414
Gross unrealized gains	5,859	7,027
Gross unrealized losses	(502)	(476)
Estimated fair value	740,378	890,965
Non-GSE | Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost		4
Gross unrealized gains		0
Gross unrealized losses		0
Estimated fair value		4
Municipal bonds | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	77	122
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Estimated fair value	78	123
Corporate bonds | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	123,678	87,319
Gross unrealized gains	815	1,099
Gross unrealized losses	(47)	0
Estimated fair value	$ 124,446	88,418
Asset-backed securities | Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost		779
Gross unrealized gains		15
Gross unrealized losses		0
Estimated fair value		$ 794

X

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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