Investment Securities - Amortized Cost and Fair Values of Held-to-Maturity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Held-to-maturity:
Amortized Cost	$ 332,533	$ 363,883
Gross Unrealized Gains	19,267	8,436
Gross Unrealized Losses	(1)	(1,014)
Estimated Fair Value	351,799	371,305
Mortgage-backed securities
Held-to-maturity:
Amortized Cost	332,533	363,883
Gross Unrealized Gains	19,267	8,436
Gross Unrealized Losses	(1)	(1,014)
Estimated Fair Value	$ 351,799	$ 371,305

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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