Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 56,356	$ 55,331	$ 55,533	$ 50,038
Charge-offs	(4,897)	(3,193)	(12,471)	(8,656)
Recoveries	1,098	852	2,754	2,089
Provision	490	5,747	7,231	15,266
Valuation allowance, balance at the end of the period	53,047	58,737	53,047	58,737
Ending balance: individually evaluated for impairment	2,868		2,868		$ 4,083
Ending balance: collectively evaluated for impairment	50,179		50,179		51,450
Financing Receivables:
Total financing receivables	4,678,144		4,678,144		4,743,619
Ending balance: individually evaluated for impairment	36,054		36,054		51,759
Ending balance: collectively evaluated for impairment	4,642,090		4,642,090		4,691,860
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	3,130	4,384	2,873	4,186
Charge-offs	(522)	(373)	(528)	(433)
Recoveries	33	92	91	144
Provision	347	154	552	360
Valuation allowance, balance at the end of the period	2,988	4,257	2,988	4,257
Ending balance: individually evaluated for impairment	1,317		1,317		1,352
Ending balance: collectively evaluated for impairment	1,671		1,671		1,521
Financing Receivables:
Total financing receivables	2,066,023		2,066,023		2,048,051
Ending balance: individually evaluated for impairment	19,757		19,757		19,854
Ending balance: collectively evaluated for impairment	2,046,266		2,046,266		2,028,197
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	18,840	13,561	16,004	11,342
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(2,800)	1,289	36	3,508
Valuation allowance, balance at the end of the period	16,040	14,850	16,040	14,850
Ending balance: individually evaluated for impairment	72		72		80
Ending balance: collectively evaluated for impairment	15,968		15,968		15,924
Financing Receivables:
Total financing receivables	745,583		745,583		800,395
Ending balance: individually evaluated for impairment	1,281		1,281		1,569
Ending balance: collectively evaluated for impairment	744,302		744,302		798,826
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	5,527	7,836	5,039	7,260
Charge-offs	0	(108)	(14)	(108)
Recoveries	164	15	294	46
Provision	(36)	(248)	336	297
Valuation allowance, balance at the end of the period	5,655	7,495	5,655	7,495
Ending balance: individually evaluated for impairment	409		409		215
Ending balance: collectively evaluated for impairment	5,246		5,246		4,824
Financing Receivables:
Total financing receivables	905,249		905,249		863,163
Ending balance: individually evaluated for impairment	7,078		7,078		6,158
Ending balance: collectively evaluated for impairment	898,171		898,171		857,005
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	1,264	1,689	1,738	1,671
Charge-offs	0	0	(92)	0
Recoveries	259	187	477	306
Provision	(141)	23	(741)	(78)
Valuation allowance, balance at the end of the period	1,382	1,899	1,382	1,899
Ending balance: individually evaluated for impairment	373		373		789
Ending balance: collectively evaluated for impairment	1,009		1,009		949
Financing Receivables:
Total financing receivables	18,611		18,611		18,889
Ending balance: individually evaluated for impairment	2,385		2,385		3,629
Ending balance: collectively evaluated for impairment	16,226		16,226		15,260
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4,706	6,993	6,449	4,461
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	370	179	(1,373)	2,711
Valuation allowance, balance at the end of the period	5,076	7,172	5,076	7,172
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	5,076		5,076		6,449
Financing Receivables:
Total financing receivables	128,407		128,407		126,768
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	128,407		128,407		126,768
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	9	12	12	13
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	2	(2)	(1)	(3)
Valuation allowance, balance at the end of the period	11	10	11	10
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	11		11		12
Financing Receivables:
Total financing receivables	13,031		13,031		16,080
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	13,031		13,031		16,080
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14,552	17,085	16,618	17,208
Charge-offs	(1,215)	(833)	(3,477)	(3,138)
Recoveries	326	347	922	907
Provision	(595)	2,457	(995)	4,079
Valuation allowance, balance at the end of the period	13,068	19,056	13,068	19,056
Ending balance: individually evaluated for impairment	667		667		1,641
Ending balance: collectively evaluated for impairment	12,401		12,401		14,977
Financing Receivables:
Total financing receivables	589,669		589,669		692,051
Ending balance: individually evaluated for impairment	5,486		5,486		20,539
Ending balance: collectively evaluated for impairment	584,183		584,183		671,512
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	8,328	3,771	6,800	3,897
Charge-offs	(3,160)	(1,879)	(8,360)	(4,977)
Recoveries	316	211	970	686
Provision	3,343	1,895	9,417	4,392
Valuation allowance, balance at the end of the period	8,827	3,998	8,827	3,998
Ending balance: individually evaluated for impairment	30		30		6
Ending balance: collectively evaluated for impairment	8,797		8,797		6,794
Financing Receivables:
Total financing receivables	211,571		211,571		178,222
Ending balance: individually evaluated for impairment	67		67		10
Ending balance: collectively evaluated for impairment	211,504		211,504		178,212
Unallocated
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	0	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	0	0	0
Valuation allowance, balance at the end of the period	0	$ 0	0	$ 0
Ending balance: collectively evaluated for impairment	$ 0		$ 0		$ 0