Investment in Securities (Schedule of Investment Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Fair Value	$ 470,523	$ 390,552
Gross Unrealized Gains	9,528	5,239
Gross Unrealized Losses	1,383	3,731
Amortized Cost	462,378	389,044
State and Political Subdivisions [Member]
Investment [Line Items]
Fair Value	417,032	370,134
Gross Unrealized Gains	9,528	5,239
Gross Unrealized Losses	661	3,328
Amortized Cost	408,165	368,223
U.S. government agencies [Member]
Investment [Line Items]
Fair Value	45,500	12,672
Gross Unrealized Gains
Gross Unrealized Losses	722	403
Amortized Cost	46,222	13,075
Certificates of Deposit [Member]
Investment [Line Items]
Fair Value	7,991	7,746
Gross Unrealized Gains
Gross Unrealized Losses
Amortized Cost	$ 7,991	$ 7,746

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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