Bank segment - Credit risk profile - summary (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Credit risk profile based on nonaccrual loans
Nonaccrual loans	$ 25,143	$ 23,591
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	22,627	20,814
Residential 1-4 family
Credit risk profile based on nonaccrual loans
Nonaccrual loans	13,578	12,598
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	10,874	10,982
Commercial real estate
Credit risk profile based on nonaccrual loans
Nonaccrual loans	0	0
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	1,004	1,016
Home equity line of credit
Credit risk profile based on nonaccrual loans
Nonaccrual loans	5,049	4,466
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	8,467	6,584
Residential land
Credit risk profile based on nonaccrual loans
Nonaccrual loans	853	841
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	331	425
Commercial construction
Credit risk profile based on nonaccrual loans
Nonaccrual loans	0	0
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	0	0
Residential construction
Credit risk profile based on nonaccrual loans
Nonaccrual loans	0	0
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	0	0
Commercial loans
Credit risk profile based on nonaccrual loans
Nonaccrual loans	2,714	3,069
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	1,886	1,741
Consumer loans
Credit risk profile based on nonaccrual loans
Nonaccrual loans	2,949	2,617
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	$ 65	$ 66