Bank segment - Credit risk profile - summary (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Credit risk profile based on nonaccrual loans
Nonaccrual loans	$ 32,028	$ 27,295
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	26,903	25,917
Residential 1-4 family
Credit risk profile based on nonaccrual loans
Nonaccrual loans	12,076	12,037
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	9,981	10,194
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	877	915
Home equity line of credit
Credit risk profile based on nonaccrual loans
Nonaccrual loans	7,859	6,348
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	10,686	11,597
Residential land
Credit risk profile based on nonaccrual loans
Nonaccrual loans	457	436
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	2,737	1,622
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	7,004	4,278
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	2,564	1,527
Consumer loans
Credit risk profile based on nonaccrual loans
Nonaccrual loans	4,632	4,196
Accruing loans 90 days or more past due	0	0
Total troubled debt restructuring loans not included above	$ 58	$ 62