Allowance for Loan Losses - Loan Portfolio and Related Allowance (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Recorded Investment
Individually Evaluated for Impairment	$ 32,608	$ 30,291
Collectively Evaluated for Impairment	3,850,824	3,769,670
Total	3,883,432	3,799,961
Associated Allowance
Individually Evaluated for Impairment	2,745	2,402
Collectively Evaluated for Impairment	25,373	24,720
Total	28,118	27,122
PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	13,693
Collectively Evaluated for Impairment		0
Associated Allowance
Individually Evaluated for Impairment		17,417
Collectively Evaluated for Impairment	0
Construction and Land Development [Member]
Recorded Investment
Individually Evaluated for Impairment	451	474
Collectively Evaluated for Impairment	372,643	341,530
Total	373,094	342,004
Associated Allowance
Individually Evaluated for Impairment	26	23
Collectively Evaluated for Impairment	2,032	1,619
Total	2,058	1,642
Construction and Land Development [Member] \| PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	1,150		$ 1,121
Associated Allowance
Individually Evaluated for Impairment	0		0
Commercial Real Estate [Member]
Recorded Investment
Individually Evaluated for Impairment	7,526	8,255
Collectively Evaluated for Impairment	1,627,858	1,621,960
Total	1,635,384	1,630,215
Associated Allowance
Individually Evaluated for Impairment	178	195
Collectively Evaluated for Impairment	8,664	9,090
Total	8,842	9,285
Commercial Real Estate [Member] \| PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	9,855		9,776
Associated Allowance
Individually Evaluated for Impairment	0		0
Residential Real Estate [Member]
Recorded Investment
Individually Evaluated for Impairment	21,900	18,720
Collectively Evaluated for Impairment	1,041,383	1,014,465
Total	1,063,283	1,033,185
Associated Allowance
Individually Evaluated for Impairment	1,094	1,091
Collectively Evaluated for Impairment	6,953	6,040
Total	8,047	7,131
Residential Real Estate [Member] \| PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	1,869		5,626
Associated Allowance
Individually Evaluated for Impairment	0		0
Commercial And Financial [Member]
Recorded Investment
Individually Evaluated for Impairment	2,363	2,455
Collectively Evaluated for Impairment	613,520	602,666
Total	615,883	605,121
Associated Allowance
Individually Evaluated for Impairment	1,406	1,050
Collectively Evaluated for Impairment	5,987	6,247
Total	7,393	7,297
Commercial And Financial [Member] \| PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	819		894
Associated Allowance
Individually Evaluated for Impairment	0		0
Consumer [Member]
Recorded Investment
Individually Evaluated for Impairment	368	387
Collectively Evaluated for Impairment	195,420	189,049
Total	195,788	189,436
Associated Allowance
Individually Evaluated for Impairment	41	43
Collectively Evaluated for Impairment	1,737	1,724
Total	1,778	$ 1,767
Consumer [Member] \| PCI Loans [Member]
Recorded Investment
Individually Evaluated for Impairment	0		0
Associated Allowance
Individually Evaluated for Impairment	$ 0		$ 0

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- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

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- References

No definition available.

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