Securities - Amortized Cost and Estimated Fair Value of Securities - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Securities held-to-maturity:
Amortized Cost	$ 388,603	$ 398,341
Gross Unrealized Gains	366	952
Gross Unrealized (Losses)	(13,649)	(5,001)
Estimated Fair Value	375,320	394,292
Agency mortgage-backed securities
Securities held-to-maturity:
Amortized Cost	300,690	309,616
Gross Unrealized Gains		6
Gross Unrealized (Losses)	(11,178)	(4,394)
Estimated Fair Value	289,512	305,228
Municipals - exempt from Federal tax
Securities held-to-maturity:
Amortized Cost	87,913	88,725
Gross Unrealized Gains	366	946
Gross Unrealized (Losses)	(2,471)	(607)
Estimated Fair Value	$ 85,808	$ 89,064

Not applicable.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

+ References

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

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- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

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