Loans and The Allowance for Loan Losses - Additional Activity in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	$ 52	$ 81
Allowance related to non-impaired loans	20,133	20,796
Total allowance for loan losses	20,185	20,877	$ 18,875	$ 18,750
Impaired loans	10,148	12,282
Non-impaired loans	2,191,188	2,191,528
Total loans	2,201,336	2,203,810
Consumer, Excluding Consumer Other Portfolio Segment \| 1-4 Family Residential
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	51	80
Allowance related to non-impaired loans	5,048	4,996
Total allowance for loan losses	5,099	5,076	5,000	4,846
Impaired loans	6,027	5,949
Non-impaired loans	928,568	916,678
Total loans	934,595	922,627
Consumer, Excluding Consumer Other Portfolio Segment \| Home Equity
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	0	0
Allowance related to non-impaired loans	631	699
Total allowance for loan losses	631	699	558	537
Impaired loans	1,327	1,387
Non-impaired loans	73,686	79,275
Total loans	75,013	80,662
Consumer, Excluding Consumer Other Portfolio Segment \| Commercial Real Estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	0	0
Allowance related to non-impaired loans	9,511	9,584
Total allowance for loan losses	9,511	9,584	8,333	8,374
Impaired loans	2,397	4,744
Non-impaired loans	846,781	829,520
Total loans	849,178	834,264
Consumer and Commercial Portfolio Segment \| Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	0	0
Allowance related to non-impaired loans	1,281	1,708
Total allowance for loan losses	1,281	1,708	1,457	1,353
Impaired loans	0	0
Non-impaired loans	73,354	91,050
Total loans	73,354	91,050
Commercial business
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	0	0
Allowance related to non-impaired loans	3,353	3,473
Total allowance for loan losses	3,353	3,473	3,126	3,206
Impaired loans	305	0
Non-impaired loans	248,980	253,509
Total loans	249,285	253,509
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance related to impaired loans	1	1
Allowance related to non-impaired loans	309	336
Total allowance for loan losses	310	337	$ 401	$ 434
Impaired loans	92	202
Non-impaired loans	19,819	21,496
Total loans	$ 19,911	$ 21,698

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

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

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Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

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Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

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