Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 55,997	$ 52,326	$ 55,533	$ 50,038
Charge-offs	(3,234)	(2,505)	(7,574)	(5,463)
Recoveries	759	757	1,656	1,237
Provision	2,834	4,753	6,741	9,519
Valuation allowance, balance at the end of the period	56,356	55,331	56,356	55,331
Ending balance: individually evaluated for impairment	3,129		3,129		$ 4,083
Ending balance: collectively evaluated for impairment	53,227		53,227		51,450
Financing Receivables:
Total financing receivables	4,747,756		4,747,756		4,743,619
Ending balance: individually evaluated for impairment	34,101		34,101		51,759
Ending balance: collectively evaluated for impairment	4,713,655		4,713,655		4,691,860
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,781	4,593	2,873	4,186
Charge-offs	0	(15)	(6)	(60)
Recoveries	49	35	58	52
Provision	300	(229)	205	206
Valuation allowance, balance at the end of the period	3,130	4,384	3,130	4,384
Ending balance: individually evaluated for impairment	1,332		1,332		1,352
Ending balance: collectively evaluated for impairment	1,798		1,798		1,521
Financing Receivables:
Total financing receivables	2,061,549		2,061,549		2,048,051
Ending balance: individually evaluated for impairment	19,188		19,188		19,854
Ending balance: collectively evaluated for impairment	2,042,361		2,042,361		2,028,197
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,504	11,806	16,004	11,342
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	2,336	1,755	2,836	2,219
Valuation allowance, balance at the end of the period	18,840	13,561	18,840	13,561
Ending balance: individually evaluated for impairment	73		73		80
Ending balance: collectively evaluated for impairment	18,767		18,767		15,924
Financing Receivables:
Total financing receivables	808,900		808,900		800,395
Ending balance: individually evaluated for impairment	1,289		1,289		1,569
Ending balance: collectively evaluated for impairment	807,611		807,611		798,826
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	5,417	7,172	5,039	7,260
Charge-offs	0	0	(14)	0
Recoveries	39	16	130	31
Provision	71	648	372	545
Valuation allowance, balance at the end of the period	5,527	7,836	5,527	7,836
Ending balance: individually evaluated for impairment	275		275		215
Ending balance: collectively evaluated for impairment	5,252		5,252		4,824
Financing Receivables:
Total financing receivables	883,135		883,135		863,163
Ending balance: individually evaluated for impairment	6,684		6,684		6,158
Ending balance: collectively evaluated for impairment	876,451		876,451		857,005
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	1,479	1,740	1,738	1,671
Charge-offs	(92)	0	(92)	0
Recoveries	15	16	218	119
Provision	(138)	(67)	(600)	(101)
Valuation allowance, balance at the end of the period	1,264	1,689	1,264	1,689
Ending balance: individually evaluated for impairment	480		480		789
Ending balance: collectively evaluated for impairment	784		784		949
Financing Receivables:
Total financing receivables	16,009		16,009		18,889
Ending balance: individually evaluated for impairment	2,589		2,589		3,629
Ending balance: collectively evaluated for impairment	13,420		13,420		15,260
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	7,257	6,164	6,449	4,461
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(2,551)	829	(1,743)	2,532
Valuation allowance, balance at the end of the period	4,706	6,993	4,706	6,993
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	4,706		4,706		6,449
Financing Receivables:
Total financing receivables	116,548		116,548		126,768
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	116,548		116,548		126,768
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	11	12	12	13
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(2)	0	(3)	(1)
Valuation allowance, balance at the end of the period	9	12	9	12
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	9		9		12
Financing Receivables:
Total financing receivables	10,759		10,759		16,080
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	10,759		10,759		16,080
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14,902	16,991	16,618	17,208
Charge-offs	(752)	(962)	(2,262)	(2,305)
Recoveries	299	425	596	560
Provision	103	631	(400)	1,622
Valuation allowance, balance at the end of the period	14,552	17,085	14,552	17,085
Ending balance: individually evaluated for impairment	939		939		1,641
Ending balance: collectively evaluated for impairment	13,613		13,613		14,977
Financing Receivables:
Total financing receivables	649,657		649,657		692,051
Ending balance: individually evaluated for impairment	4,283		4,283		20,539
Ending balance: collectively evaluated for impairment	645,374		645,374		671,512
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	7,646	3,848	6,800	3,897
Charge-offs	(2,390)	(1,528)	(5,200)	(3,098)
Recoveries	357	265	654	475
Provision	2,715	1,186	6,074	2,497
Valuation allowance, balance at the end of the period	8,328	3,771	8,328	3,771
Ending balance: individually evaluated for impairment	30		30		6
Ending balance: collectively evaluated for impairment	8,298		8,298		6,794
Financing Receivables:
Total financing receivables	201,199		201,199		178,222
Ending balance: individually evaluated for impairment	68		68		10
Ending balance: collectively evaluated for impairment	201,131		201,131		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