Debt Securities Available-for-Sale - Comparative Summary of Mortgage-Backed Securities and Other Securities Available-for-Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 898,434	$ 820,801
Gross unrealized gains	3,952	967
Gross unrealized losses	8,114	13,737
Debt securities available-for-sale	894,272	808,031
Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	672,565	575,639
Gross unrealized gains	3,221	892
Gross unrealized losses	7,700	11,522
Debt securities available-for-sale	668,086	565,009
Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	225,869	245,162
Gross unrealized gains	731	75
Gross unrealized losses	414	2,215
Debt securities available-for-sale	226,186	243,022
GSE \| Pass-through Certificates
Debt Securities, Available-for-sale [Line Items]
Amortized cost	404,108	317,530
Gross unrealized gains	2,656	800
Gross unrealized losses	2,227	3,542
Debt securities available-for-sale	404,537	314,788
GSE \| Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	268,399	258,050
Gross unrealized gains	565	92
Gross unrealized losses	5,472	7,979
Debt securities available-for-sale	263,492	250,163
Non-GSE \| Real estate mortgage investment conduits (REMICs)
Debt Securities, Available-for-sale [Line Items]
Amortized cost	58	59
Gross unrealized gains	0	0
Gross unrealized losses	1	1
Debt securities available-for-sale	57	58
Municipal bonds \| Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	270	270
Gross unrealized gains	4	3
Gross unrealized losses	0	0
Debt securities available-for-sale	274	273
Corporate bonds \| Other debt obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	225,599	244,892
Gross unrealized gains	727	72
Gross unrealized losses	414	2,215
Debt securities available-for-sale	$ 225,912	$ 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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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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).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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