Bank segment - Loan modifications (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) contract	Mar. 31, 2017 USD ($) contract	Dec. 31, 2017 USD ($)
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	$ 22,627		$ 20,814
Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	10,874		10,982
Commercial real estate
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	1,004		1,016
Home equity line of credit
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	8,467		6,584
Residential land
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	331		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	1,886		1,741
Consumer loans
Financing Receivable, Modifications [Line Items]
Post-modification outstanding recorded investment	$ 65		$ 66
Troubled debt restructurings real estate loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	25	13
Pre-modification outstanding recorded investment	$ 4,869	$ 1,139
Post-modification outstanding recorded investment	4,878	1,133
Net increase in allowance	$ 404	$ 106
Troubled debt restructurings real estate loans \| Residential 1-4 family
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	1	3
Pre-modification outstanding recorded investment	$ 339	$ 512
Post-modification outstanding recorded investment	344	520
Net increase in allowance	$ 16	$ 45
Troubled debt restructurings real estate loans \| Commercial real estate
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0
Post-modification outstanding recorded investment	0	0
Net increase in allowance	$ 0	$ 0
Troubled debt restructurings real estate loans \| Home equity line of credit
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	18	8
Pre-modification outstanding recorded investment	$ 2,170	$ 226
Post-modification outstanding recorded investment	2,174	212
Net increase in allowance	$ 388	$ 34
Troubled debt restructurings real estate loans \| Residential land
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	1	0
Pre-modification outstanding recorded investment	$ 109	$ 0
Post-modification outstanding recorded investment	109	0
Net increase in allowance	$ 0	$ 0
Troubled debt restructurings real estate loans \| Commercial construction
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0
Post-modification outstanding recorded investment	0	0
Net increase in allowance	$ 0	$ 0
Troubled debt restructurings real estate loans \| Residential construction
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	0
Pre-modification outstanding recorded investment	$ 0	$ 0
Post-modification outstanding recorded investment	0	0
Net increase in allowance	$ 0	$ 0
Troubled debt restructurings real estate loans \| Commercial loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	5	1
Pre-modification outstanding recorded investment	$ 2,251	$ 342
Post-modification outstanding recorded investment	2,251	342
Net increase in allowance	$ 0	$ 0
Troubled debt restructurings real estate loans \| Consumer loans
Financing Receivable, Modifications [Line Items]
Number of contracts \| contract	0	1
Pre-modification outstanding recorded investment	$ 0	$ 59
Post-modification outstanding recorded investment	0	59
Net increase in allowance	$ 0	$ 27

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

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