Bank segment - Loan modifications (Details) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($) contract	Sep. 30, 2018 USD ($) contract	Sep. 30, 2017 USD ($) contract	Dec. 31, 2017 USD ($)
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 26,224		$ 20,814
Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			10,701		10,982
Commercial real estate
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			981		1,016
Home equity line of credit
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			11,131		6,584
Residential land
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			1,542		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			1,806		1,741
Consumer loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment			$ 63		$ 66
Troubled debt restructurings real estate loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	28	18	76	38
Pre-modification outstanding recorded investment	$ 4,034	$ 1,275	$ 12,284	$ 3,072
Post-modification outstanding recorded investment	4,058	1,275	12,317	3,066
Net increase in allowance	$ 398	$ 222	$ 1,398	$ 387
Troubled debt restructurings real estate loans \| Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	3	2	4	7
Pre-modification outstanding recorded investment	$ 632	$ 83	$ 971	$ 955
Post-modification outstanding recorded investment	649	83	993	963
Net increase in allowance	$ 1	$ 0	$ 17	$ 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	16	15	55	28
Pre-modification outstanding recorded investment	$ 1,584	$ 862	$ 7,092	$ 1,386
Post-modification outstanding recorded investment	1,585	862	7,097	1,372
Net increase in allowance	$ 263	$ 184	$ 1,205	$ 277
Troubled debt restructurings real estate loans \| Residential land
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	3	0	4	0
Pre-modification outstanding recorded investment	$ 1,562	$ 0	$ 1,671	$ 0
Post-modification outstanding recorded investment	1,568	0	1,677	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	6	1	13	2
Pre-modification outstanding recorded investment	$ 256	$ 330	$ 2,550	$ 672
Post-modification outstanding recorded investment	256	330	2,550	672
Net increase in allowance	$ 134	$ 38	$ 176	$ 38
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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