Bank segment - Principal balance of impaired loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Recorded investment:
With no related allowance recorded	$ 17,085		$ 17,085		$ 14,072
With an allowance recorded	24,075		24,075		19,321
Recorded investment	41,160		41,160		33,393
Unpaid principal balance:
With no related allowance recorded	19,117		19,117		16,041
With an allowance recorded	24,363		24,363		19,574
Unpaid principal balance	43,480		43,480		35,615
Related Allowance	2,890		2,890		2,730
Average recorded investment:
With no related allowance recorded	16,895	$ 14,585	15,357	$ 15,700
With an allowance recorded	23,746	19,948	21,916	21,704
Average recorded investment	40,641	34,533	37,273	37,404
Interest income recognized:
With no related allowance recorded	274	204	473	482
With an allowance recorded	235	416	694	1,307
Interest income recognized	509	620	1,167	1,789
Residential 1-4 family
Recorded investment:
With no related allowance recorded	8,689		8,689		9,097
With an allowance recorded	9,014		9,014		9,187
Recorded investment	17,703		17,703		18,284
Unpaid principal balance:
With no related allowance recorded	9,200		9,200		9,644
With an allowance recorded	9,218		9,218		9,390
Unpaid principal balance	18,418		18,418		19,034
Related Allowance	1,020		1,020		1,248
Average recorded investment:
With no related allowance recorded	8,940	9,650	8,779	9,503
With an allowance recorded	8,820	9,788	8,909	9,963
Average recorded investment	17,760	19,438	17,688	19,466
Interest income recognized:
With no related allowance recorded	239	70	396	230
With an allowance recorded	84	97	274	333
Interest income recognized	323	167	670	563
Commercial real estate
Recorded investment:
With no related allowance recorded	0		0		0
With an allowance recorded	981		981		1,016
Recorded investment	981		981		1,016
Unpaid principal balance:
With no related allowance recorded	0		0		0
With an allowance recorded	981		981		1,016
Unpaid principal balance	981		981		1,016
Related Allowance	51		51		65
Average recorded investment:
With no related allowance recorded	0	0	0	121
With an allowance recorded	985	1,284	997	1,292
Average recorded investment	985	1,284	997	1,413
Interest income recognized:
With no related allowance recorded	0	0	0	11
With an allowance recorded	11	13	32	41
Interest income recognized	11	13	32	52
Home equity line of credit
Recorded investment:
With no related allowance recorded	2,359		2,359		1,496
With an allowance recorded	12,243		12,243		6,692
Recorded investment	14,602		14,602		8,188
Unpaid principal balance:
With no related allowance recorded	2,714		2,714		1,789
With an allowance recorded	12,327		12,327		6,736
Unpaid principal balance	15,041		15,041		8,525
Related Allowance	1,088		1,088		647
Average recorded investment:
With no related allowance recorded	2,234	1,918	2,103	2,108
With an allowance recorded	12,090	5,076	10,083	4,670
Average recorded investment	14,324	6,994	12,186	6,778
Interest income recognized:
With no related allowance recorded	23	32	35	97
With an allowance recorded	111	68	288	164
Interest income recognized	134	100	323	261
Residential land
Recorded investment:
With no related allowance recorded	2,057		2,057		1,143
With an allowance recorded	0		0		122
Recorded investment	2,057		2,057		1,265
Unpaid principal balance:
With no related allowance recorded	2,256		2,256		1,434
With an allowance recorded	0		0		122
Unpaid principal balance	2,256		2,256		1,556
Related Allowance	0		0		47
Average recorded investment:
With no related allowance recorded	1,773	1,209	1,358	1,080
With an allowance recorded	20	1,251	45	1,620
Average recorded investment	1,793	2,460	1,403	2,700
Interest income recognized:
With no related allowance recorded	6	73	16	107
With an allowance recorded	0	12	3	73
Interest income recognized	6	85	19	180
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	3,948		3,948		2,328
With an allowance recorded	1,779		1,779		2,246
Recorded investment	5,727		5,727		4,574
Unpaid principal balance:
With no related allowance recorded	4,915		4,915		3,166
With an allowance recorded	1,779		1,779		2,252
Unpaid principal balance	6,694		6,694		5,418
Related Allowance	728		728		694
Average recorded investment:
With no related allowance recorded	3,915	1,808	3,099	2,888
With an allowance recorded	1,774	2,482	1,824	4,104
Average recorded investment	5,689	4,290	4,923	6,992
Interest income recognized:
With no related allowance recorded	6	29	26	37
With an allowance recorded	28	225	94	694
Interest income recognized	34	254	120	731
Consumer loans
Recorded investment:
With no related allowance recorded	32		32		8
With an allowance recorded	58		58		58
Recorded investment	90		90		66
Unpaid principal balance:
With no related allowance recorded	32		32		8
With an allowance recorded	58		58		58
Unpaid principal balance	90		90		66
Related Allowance	3		3		$ 29
Average recorded investment:
With no related allowance recorded	33	0	18	0
With an allowance recorded	57	67	58	55
Average recorded investment	90	67	76	55
Interest income recognized:
With no related allowance recorded	0	0	0	0
With an allowance recorded	1	1	3	2
Interest income recognized	$ 1	$ 1	$ 3	$ 2