Bank segment - Credit risk profile - payment activity (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Credit risk profile based on payment activity for loans
Total past due	$ 20,756	$ 23,603
Current	4,721,924	4,647,974
Total financing receivables	4,742,680	4,671,577
Recorded Investment greater than 90 days and accruing	0	0
30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	7,078	7,237
60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	4,224	6,013
Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	9,454	10,353
Residential 1-4 family
Credit risk profile based on payment activity for loans
Total past due	7,169	8,318
Current	2,108,952	2,109,729
Total financing receivables	2,116,121	2,118,047
Recorded Investment greater than 90 days and accruing	0	0
Residential 1-4 family | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	1,902	1,532
Residential 1-4 family | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	662	1,715
Residential 1-4 family | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	4,605	5,071
Commercial real estate
Credit risk profile based on payment activity for loans
Total past due	0	0
Current	766,522	733,106
Total financing receivables	766,522	733,106
Recorded Investment greater than 90 days and accruing	0	0
Commercial real estate | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Commercial real estate | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Commercial real estate | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	0	0
Home equity line of credit
Credit risk profile based on payment activity for loans
Total past due	5,414	2,590
Current	909,527	910,462
Total financing receivables	914,941	913,052
Recorded Investment greater than 90 days and accruing	0	0
Home equity line of credit | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	1,943	425
Home equity line of credit | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	1,350	114
Home equity line of credit | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	2,121	2,051
Residential land
Credit risk profile based on payment activity for loans
Total past due	640	648
Current	15,929	15,149
Total financing receivables	16,569	15,797
Recorded Investment greater than 90 days and accruing	0	0
Residential land | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	0	23
Residential land | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Residential land | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	640	625
Commercial construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Current	114,535	108,273
Total financing receivables	114,535	108,273
Recorded Investment greater than 90 days and accruing	0	0
Commercial construction | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Commercial construction | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Commercial construction | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	0	0
Residential construction
Credit risk profile based on payment activity for loans
Total past due	0	0
Current	15,363	14,910
Total financing receivables	15,363	14,910
Recorded Investment greater than 90 days and accruing	0	0
Residential construction | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Residential construction | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	0	0
Residential construction | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	0	0
Commercial loans
Credit risk profile based on payment activity for loans
Total past due	1,265	4,580
Current	567,106	540,248
Total financing receivables	568,371	544,828
Recorded Investment greater than 90 days and accruing	0	0
Commercial loans | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	344	1,825
Commercial loans | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	689	2,025
Commercial loans | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	232	730
Consumer loans
Credit risk profile based on payment activity for loans
Total past due	6,268	7,467
Current	223,990	216,097
Total financing receivables	230,258	223,564
Recorded Investment greater than 90 days and accruing	0	0
Consumer loans | 30-59 days past due
Credit risk profile based on payment activity for loans
Total past due	2,889	3,432
Consumer loans | 60-89 days past due
Credit risk profile based on payment activity for loans
Total past due	1,523	2,159
Consumer loans | Greater than 90 days
Credit risk profile based on payment activity for loans
Total past due	$ 1,856	$ 1,876