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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, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,017,254	$ 1,131,970
Gross unrealized gains	20,437	9,374
Gross unrealized losses	202	2,992
Debt securities available-for-sale	1,037,489	1,138,352
Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	924,370	967,949
Gross unrealized gains	19,041	8,157
Gross unrealized losses	202	2,979
Debt securities available-for-sale	943,209	973,127
Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	92,884	164,021
Gross unrealized gains	1,396	1,217
Gross unrealized losses	0	13
Debt securities available-for-sale	94,280	165,225
GSE | Pass-through Certificates
Debt Securities, Available-for-sale [Line Items]
Amortized cost	278,065	324,080
Gross unrealized gains	11,163	6,081
Gross unrealized losses	75	754
Debt securities available-for-sale	289,153	329,407
GSE | Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	646,257	643,816
Gross unrealized gains	7,878	2,076
Gross unrealized losses	126	2,225
Debt securities available-for-sale	654,009	643,667
Non-GSE | Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	48	53
Gross unrealized gains	0	0
Gross unrealized losses	1	0
Debt securities available-for-sale	47	53
Municipal bonds | Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	253	296
Gross unrealized gains	1	3
Gross unrealized losses	0	0
Debt securities available-for-sale	254	299
Corporate bonds | Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	92,631	163,725
Gross unrealized gains	1,395	1,214
Gross unrealized losses	0	13
Debt securities available-for-sale	$ 94,026	$ 164,926

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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