Investments - Amortized Cost and Estimated Fair Value of Available-For-Sale Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 566,930	$ 569,942
Gross Unrealized Gains	3,554	1,616
Gross Unrealized Losses	(6,856)	(11,079)
Estimated Fair Value	563,628	560,479
US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,623	15,553
Gross Unrealized Gains	44	12
Gross Unrealized Losses	(220)	(353)
Estimated Fair Value	15,447	15,212
Mortgage-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	407,824	414,208
Gross Unrealized Gains	994	398
Gross Unrealized Losses	(6,349)	(9,873)
Estimated Fair Value	402,469	404,733
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	143,483	140,181
Gross Unrealized Gains	2,516	1,206
Gross Unrealized Losses	(287)	(853)
Estimated Fair Value	$ 145,712	$ 140,534

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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