Bank segment - Credit risk profile - summary (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Credit risk profile based on nonaccrual loans
Nonaccrual loans	$ 23,357	$ 23,325
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	21,797	37,637
Residential 1-4 family
Credit risk profile based on nonaccrual loans
Nonaccrual loans	12,853	11,154
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	11,592	14,450
Commercial real estate
Credit risk profile based on nonaccrual loans
Nonaccrual loans	0	223
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	1,281	1,346
Home equity line of credit
Credit risk profile based on nonaccrual loans
Nonaccrual loans	4,000	3,080
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	5,250	4,934
Residential land
Credit risk profile based on nonaccrual loans
Nonaccrual loans	1,022	878
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	1,555	2,751
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	3,691	6,708
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	2,052	14,146
Consumer loans
Credit risk profile based on nonaccrual loans
Nonaccrual loans	1,791	1,282
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	$ 67	$ 10