Bank segment - Credit risk profile - payment activity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Credit risk profile based on payment activity for loans
Financing receivable, past due	$ 24,635	$ 19,751
Financing receivable, current	5,155,743	5,100,913
Ending balance	5,180,378	5,120,664
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	53,548
Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	11,390	8,864
Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	4,781	3,858
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	8,464	7,029
Residential 1-4 family
Credit risk profile based on payment activity for loans
Financing receivable, past due	5,687	4,686
Financing receivable, current	2,156,207	2,173,449
Ending balance	2,161,894	2,178,135
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	0
Residential 1-4 family | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	2,781	2,588
Revolving loans converted to term loans	0
Residential 1-4 family | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	1,630	290
Revolving loans converted to term loans	0
Residential 1-4 family | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	1,276	1,808
Revolving loans converted to term loans	0
Commercial real estate
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Financing receivable, current	852,120	824,830
Ending balance	852,120	824,830
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	0
Commercial real estate | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Commercial real estate | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Commercial real estate | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Home equity line of credit
Credit risk profile based on payment activity for loans
Financing receivable, past due	3,253	2,930
Financing receivable, current	1,092,424	1,089,195
Ending balance	1,095,677	1,092,125
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	31,546
Home equity line of credit | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	1,225	813
Revolving loans converted to term loans	592
Home equity line of credit | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	224	0
Revolving loans converted to term loans	198
Home equity line of credit | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	1,804	2,117
Revolving loans converted to term loans	429
Residential land
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	25
Financing receivable, current	13,720	14,679
Ending balance	13,720	14,704
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	0
Residential land | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Revolving loans converted to term loans	0
Residential land | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Revolving loans converted to term loans	0
Residential land | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	25
Revolving loans converted to term loans	0
Commercial construction
Credit risk profile based on payment activity for loans
Financing receivable, past due	2,289	0
Financing receivable, current	77,088	70,605
Ending balance	79,377	70,605
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	0
Commercial construction | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Commercial construction | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Commercial construction | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	2,289	0
Residential construction
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Financing receivable, current	9,190	11,670
Ending balance	9,190	11,670
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	0
Residential construction | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Revolving loans converted to term loans	0
Residential construction | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Revolving loans converted to term loans	0
Residential construction | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	0	0
Revolving loans converted to term loans	0
Commercial loans
Credit risk profile based on payment activity for loans
Financing receivable, past due	4,332	1,560
Financing receivable, current	718,315	669,114
Ending balance	722,647	670,674
Recorded Investment greater than 90 days and accruing	0	0
Revolving loans converted to term loans	18,778
Commercial loans | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	3,712	1,077
Commercial loans | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	311	311
Commercial loans | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	309	172
Consumer loans
Credit risk profile based on payment activity for loans
Financing receivable, past due	9,074	10,550
Financing receivable, current	236,679	247,371
Ending balance	245,753	257,921
Recorded Investment greater than 90 days and accruing	0	0
Consumer loans | Financial Asset, 30 to 59 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	3,672	4,386
Consumer loans | Financial Asset, 60 to 89 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	2,616	3,257
Consumer loans | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Credit risk profile based on payment activity for loans
Financing receivable, past due	$ 2,786	$ 2,907