Bank segment - Loan modifications (Details) - Troubled debt restructurings real estate loans $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract	Sep. 30, 2019 USD ($) contract	Sep. 30, 2018 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	5	26	22	69
Outstanding recorded investment	$ 949	$ 3,596	$ 4,922	$ 10,691
Net increase in allowance	$ 58	$ 476	$ 468	$ 1,167
Residential 1-4 family
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	1	2	10	2
Outstanding recorded investment	$ 324	$ 427	$ 1,563	$ 427
Net increase in allowance	$ 0	$ 19	$ 165	$ 19
Commercial real estate
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	0	0	0	0
Outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Home equity line of credit
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	0	16	3	52
Outstanding recorded investment	$ 0	$ 1,571	$ 429	$ 6,540
Net increase in allowance	$ 0	$ 283	$ 85	$ 930
Residential land
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	1	2	3	2
Outstanding recorded investment	$ 350	$ 1,343	$ 1,169	$ 1,343
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Commercial construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	0	0	0	0
Outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Residential construction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	0	0	0	0
Outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0
Commercial loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	3	6	6	13
Outstanding recorded investment	$ 275	$ 255	$ 1,761	$ 2,381
Net increase in allowance	$ 58	$ 174	$ 218	$ 218
Consumer loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of contracts \| contract	0	0	0	0
Outstanding recorded investment	$ 0	$ 0	$ 0	$ 0
Net increase in allowance	$ 0	$ 0	$ 0	$ 0

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://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

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Amount, before troubled debt restructuring, of financing receivable to be modified.

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