Bank segment - Loan modifications (Details) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract	Jun. 30, 2018 USD ($) contract	Jun. 30, 2017 USD ($) contract	Dec. 31, 2017 USD ($)
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 24,133		$ 20,814
Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			10,777		10,982
Commercial real estate
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			993		1,016
Home equity line of credit
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			10,255		6,584
Residential land
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			328		425
Commercial construction
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			0		0
Residential construction
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			0		0
Commercial loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			1,716		1,741
Consumer loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 64		$ 66
Troubled debt restructurings real estate loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	23	7	48	20
Pre-modification outstanding recorded investment	$ 3,381	$ 658	$ 8,250	$ 1,797
Post-modification outstanding recorded investment	3,381	658	8,259	1,791
Net increase in allowance	$ 596	$ 59	$ 1,000	$ 165
Troubled debt restructurings real estate loans \| Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	2	1	5
Pre-modification outstanding recorded investment	$ 0	$ 360	$ 339	$ 872
Post-modification outstanding recorded investment	0	360	344	880
Net increase in allowance	$ 0	$ 0	$ 16	$ 45
Troubled debt restructurings real estate loans \| Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans \| Home equity line of credit
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	21	5	39	13
Pre-modification outstanding recorded investment	$ 3,338	$ 298	$ 5,508	$ 524
Post-modification outstanding recorded investment	3,338	298	5,512	510
Net increase in allowance	$ 554	$ 59	$ 942	$ 93
Troubled debt restructurings real estate loans \| Residential land
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0	1	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 109	$ 0
Post-modification outstanding recorded investment	0	0	109	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans \| Commercial construction
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans \| Residential construction
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Post-modification outstanding recorded investment	0	0	0	0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings real estate loans \| Commercial loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	2	0	7	1
Pre-modification outstanding recorded investment	$ 43	$ 0	$ 2,294	$ 342
Post-modification outstanding recorded investment	43	0	2,294	342
Net increase in allowance	$ 42	$ 0	$ 42	$ 0
Troubled debt restructurings real estate loans \| Consumer loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0	0	1
Pre-modification outstanding recorded investment	$ 0	$ 0	$ 0	$ 59
Post-modification outstanding recorded investment	0	0	0	59
Net increase in allowance	$ 0	$ 0	$ 0	$ 27

Amount of increase (decrease) of allowance reserve for credit loss.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
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