Bank segment - Principal balance of impaired loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Recorded investment:
With no related allowance recorded	$ 16,803		$ 16,803		$ 14,072
With an allowance recorded	22,780		22,780		19,321
Recorded investment	39,583		39,583		33,393
Unpaid principal balance:
With no related allowance recorded	18,862		18,862		16,041
With an allowance recorded	23,053		23,053		19,574
Unpaid principal balance	41,915		41,915		35,615
Related Allowance	2,848		2,848		2,730
Average recorded investment:
With no related allowance recorded	15,447	$ 14,873	14,588	$ 16,258
With an allowance recorded	22,031	19,880	21,000	22,582
Average recorded investment	37,478	34,753	35,588	38,840
Interest income recognized:
With no related allowance recorded	72	148	199	278
With an allowance recorded	235	271	459	891
Interest income recognized	307	419	658	1,169
Residential 1-4 family
Recorded investment:
With no related allowance recorded	8,966		8,966		9,097
With an allowance recorded	8,639		8,639		9,187
Recorded investment	17,605		17,605		18,284
Unpaid principal balance:
With no related allowance recorded	9,498		9,498		9,644
With an allowance recorded	8,842		8,842		9,390
Unpaid principal balance	18,340		18,340		19,034
Related Allowance	1,027		1,027		1,248
Average recorded investment:
With no related allowance recorded	8,900	9,304	8,699	9,429
With an allowance recorded	8,778	10,054	8,953	10,051
Average recorded investment	17,678	19,358	17,652	19,480
Interest income recognized:
With no related allowance recorded	50	76	157	160
With an allowance recorded	97	117	190	236
Interest income recognized	147	193	347	396
Commercial real estate
Recorded investment:
With no related allowance recorded	0		0		0
With an allowance recorded	993		993		1,016
Recorded investment	993		993		1,016
Unpaid principal balance:
With no related allowance recorded	0		0		0
With an allowance recorded	993		993		1,016
Unpaid principal balance	993		993		1,016
Related Allowance	53		53		65
Average recorded investment:
With no related allowance recorded	0	143	0	182
With an allowance recorded	997	1,292	1,003	1,296
Average recorded investment	997	1,435	1,003	1,478
Interest income recognized:
With no related allowance recorded	0	11	0	11
With an allowance recorded	10	14	21	28
Interest income recognized	10	25	21	39
Home equity line of credit
Recorded investment:
With no related allowance recorded	2,505		2,505		1,496
With an allowance recorded	11,344		11,344		6,692
Recorded investment	13,849		13,849		8,188
Unpaid principal balance:
With no related allowance recorded	2,803		2,803		1,789
With an allowance recorded	11,414		11,414		6,736
Unpaid principal balance	14,217		14,217		8,525
Related Allowance	1,161		1,161		647
Average recorded investment:
With no related allowance recorded	2,374	2,401	2,037	2,203
With an allowance recorded	10,420	4,372	9,080	4,467
Average recorded investment	12,794	6,773	11,117	6,670
Interest income recognized:
With no related allowance recorded	7	51	12	65
With an allowance recorded	96	47	177	96
Interest income recognized	103	98	189	161
Residential land
Recorded investment:
With no related allowance recorded	1,141		1,141		1,143
With an allowance recorded	30		30		122
Recorded investment	1,171		1,171		1,265
Unpaid principal balance:
With no related allowance recorded	1,449		1,449		1,434
With an allowance recorded	30		30		122
Unpaid principal balance	1,479		1,479		1,556
Related Allowance	7		7		47
Average recorded investment:
With no related allowance recorded	1,132	1,075	1,150	1,016
With an allowance recorded	40	1,532	58	1,804
Average recorded investment	1,172	2,607	1,208	2,820
Interest income recognized:
With no related allowance recorded	5	8	10	34
With an allowance recorded	1	24	3	61
Interest income recognized	6	32	13	95
Commercial construction
Recorded investment:
With no related allowance recorded	0		0		0
With an allowance recorded	0		0		0
Recorded investment	0		0		0
Unpaid principal balance:
With no related allowance recorded	0		0		0
With an allowance recorded	0		0		0
Unpaid principal balance	0		0		0
Related Allowance	0		0		0
Average recorded investment:
With no related allowance recorded	0	0	0	0
With an allowance recorded	0	0	0	0
Average recorded investment	0	0	0	0
Interest income recognized:
With no related allowance recorded	0	0	0	0
With an allowance recorded	0	0	0	0
Interest income recognized	0	0	0	0
Residential construction
Recorded investment:
With no related allowance recorded	0		0		0
With an allowance recorded	0		0		0
Recorded investment	0		0		0
Unpaid principal balance:
With no related allowance recorded	0		0		0
With an allowance recorded	0		0		0
Unpaid principal balance	0		0		0
Related Allowance	0		0		0
Average recorded investment:
With no related allowance recorded	0	0	0	0
With an allowance recorded	0	0	0	0
Average recorded investment	0	0	0	0
Interest income recognized:
With no related allowance recorded	0	0	0	0
With an allowance recorded	0	0	0	0
Interest income recognized	0	0	0	0
Commercial loans
Recorded investment:
With no related allowance recorded	4,158		4,158		2,328
With an allowance recorded	1,716		1,716		2,246
Recorded investment	5,874		5,874		4,574
Unpaid principal balance:
With no related allowance recorded	5,079		5,079		3,166
With an allowance recorded	1,716		1,716		2,252
Unpaid principal balance	6,795		6,795		5,418
Related Allowance	597		597		694
Average recorded investment:
With no related allowance recorded	3,026	1,949	2,691	3,428
With an allowance recorded	1,738	2,562	1,848	4,915
Average recorded investment	4,764	4,511	4,539	8,343
Interest income recognized:
With no related allowance recorded	10	2	20	8
With an allowance recorded	30	68	66	469
Interest income recognized	40	70	86	477
Consumer loans
Recorded investment:
With no related allowance recorded	33		33		8
With an allowance recorded	58		58		58
Recorded investment	91		91		66
Unpaid principal balance:
With no related allowance recorded	33		33		8
With an allowance recorded	58		58		58
Unpaid principal balance	91		91		66
Related Allowance	3		3		$ 29
Average recorded investment:
With no related allowance recorded	15	1	11	0
With an allowance recorded	58	68	58	49
Average recorded investment	73	69	69	49
Interest income recognized:
With no related allowance recorded	0	0	0	0
With an allowance recorded	1	1	2	1
Interest income recognized	$ 1	$ 1	$ 2	$ 1