Debt Securities Available-for-Sale - Comparative Summary of Mortgage-Backed Securities and Other Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,131,970	$ 820,801
Gross unrealized gains	9,374	967
Gross unrealized losses	2,992	13,737
Estimated fair value	1,138,352	808,031
Pass-through certificates \| GSE
Debt Securities, Available-for-sale [Line Items]
Amortized cost	324,080	317,530
Gross unrealized gains	6,081	800
Gross unrealized losses	754	3,542
Estimated fair value	329,407	314,788
REMICs
Debt Securities, Available-for-sale [Line Items]
Amortized cost	967,949	575,639
Gross unrealized gains	8,157	892
Gross unrealized losses	2,979	11,522
Estimated fair value	973,127	565,009
REMICs \| GSE
Debt Securities, Available-for-sale [Line Items]
Amortized cost	643,816	258,050
Gross unrealized gains	2,076	92
Gross unrealized losses	2,225	7,979
Estimated fair value	643,667	250,163
REMICs \| Non-GSE
Debt Securities, Available-for-sale [Line Items]
Amortized cost	53	59
Gross unrealized gains	0	0
Gross unrealized losses	0	1
Estimated fair value	53	58
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	164,021	245,162
Gross unrealized gains	1,217	75
Gross unrealized losses	13	2,215
Estimated fair value	165,225	243,022
Other debt securities \| Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	296	270
Gross unrealized gains	3	3
Gross unrealized losses	0	0
Estimated fair value	299	273
Other debt securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost	163,725	244,892
Gross unrealized gains	1,214	72
Gross unrealized losses	13	2,215
Estimated fair value	$ 164,926	$ 242,749

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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-Topic 320
-SubTopic 10
-Section 50
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Amount, before tax, of unrealized loss 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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Amortized cost of 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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-Name Accounting Standards Codification
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Amount of 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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-Publisher FASB
-Name Accounting Standards Codification
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