Debt Securities Available-for-Sale - Comparative Summary of Mortgage-Backed Securities and Other Securities Available-for-Sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,151,568	$ 1,131,970
Gross unrealized gains	20,332	9,374
Gross unrealized losses	470	2,992
Debt securities available-for-sale	1,171,430	1,138,352
U.S. Government Agency Securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	3,168
Gross unrealized gains	2
Gross unrealized losses	0
Debt securities available-for-sale	3,170
Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	1,047,483	967,949
Gross unrealized gains	18,982	8,157
Gross unrealized losses	453	2,979
Debt securities available-for-sale	1,066,012	973,127
Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	100,917	164,021
Gross unrealized gains	1,348	1,217
Gross unrealized losses	17	13
Debt securities available-for-sale	102,248	165,225
GSE \| Pass-through Certificates
Debt Securities, Available-for-sale [Line Items]
Amortized cost	300,937	324,080
Gross unrealized gains	11,297	6,081
Gross unrealized losses	183	754
Debt securities available-for-sale	312,051	329,407
GSE \| Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	746,540	643,816
Gross unrealized gains	7,685	2,076
Gross unrealized losses	270	2,225
Debt securities available-for-sale	753,955	643,667
Non-GSE \| Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	6	53
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Debt securities available-for-sale	6	53
Municipal bonds \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	228	296
Gross unrealized gains	1	3
Gross unrealized losses	0	0
Debt securities available-for-sale	229	299
Corporate bonds \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	99,855	163,725
Gross unrealized gains	1,337	1,214
Gross unrealized losses	17	13
Debt securities available-for-sale	101,175	$ 164,926
Asset-backed securities \| Other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	834
Gross unrealized gains	10
Gross unrealized losses	0
Debt securities available-for-sale	$ 844

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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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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