Investment Securities (Summary of Amortized Cost and Fair Value of Held-to-Maturity Investment Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Securities Held-to-Maturity
Amortized Cost	$ 677,093	$ 623,427
Unrecognized Gains	3,856	2,648
Unrecognized Losses	18,043	17,544
Fair Value	662,906	608,531
State and political subdivisions
Investment Securities Held-to-Maturity
Amortized Cost	676,593	622,927
Unrecognized Gains	3,856	2,648
Unrecognized Losses	17,933	17,354
Fair Value	662,516	608,221
Trust preferred securities
Investment Securities Held-to-Maturity
Amortized Cost	500	500
Unrecognized Gains	0	0
Unrecognized Losses	110	190
Fair Value	$ 390	$ 310

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

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- Definition

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

+ References

+ Details

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- Definition

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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- Details

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