Securities - Amortized Cost and Estimated Fair Value of Securities - Securities Held-to-maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Securities held-to-maturity:
Amortized Cost	$ 398,341	$ 324,010
Gross Unrealized Gains	952	536
Gross Unrealized Losses	(5,001)	(5,798)
Estimated Fair Value	394,292	318,748
Agency mortgage-backed securities
Securities held-to-maturity:
Amortized Cost	309,616	233,409
Gross Unrealized Gains	6	15
Gross Unrealized Losses	(4,394)	(3,554)
Estimated Fair Value	305,228	229,870
Municipals - exempt from Federal tax
Securities held-to-maturity:
Amortized Cost	88,725	90,601
Gross Unrealized Gains	946	521
Gross Unrealized Losses	(607)	(2,244)
Estimated Fair Value	$ 89,064	$ 88,878

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

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

+ References

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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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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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