Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 52,119	$ 53,637
Charge-offs	(6,191)	(4,873)
Recoveries	1,499	1,590
Provision	6,870	3,541
Valuation allowance, balance at the end of the period	54,297	53,895
Ending balance: individually evaluated for impairment	4,242		$ 2,222
Ending balance: collectively evaluated for impairment	50,055		49,897
Financing Receivables:
Total financing receivables	4,858,270		4,843,634
Ending balance: individually evaluated for impairment	50,399		39,697
Ending balance: collectively evaluated for impairment	4,807,871		4,803,937
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	1,976	2,902
Charge-offs	(14)	(31)
Recoveries	609	54
Provision	(660)	(400)
Valuation allowance, balance at the end of the period	1,911	2,525
Ending balance: individually evaluated for impairment	771		876
Ending balance: collectively evaluated for impairment	1,140		1,100
Financing Receivables:
Total financing receivables	2,159,886		2,143,397
Ending balance: individually evaluated for impairment	17,403		16,494
Ending balance: collectively evaluated for impairment	2,142,483		2,126,903
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14,505	15,796
Charge-offs	0	0
Recoveries	0	0
Provision	320	163
Valuation allowance, balance at the end of the period	14,825	15,959
Ending balance: individually evaluated for impairment	7		7
Ending balance: collectively evaluated for impairment	14,818		14,498
Financing Receivables:
Total financing receivables	737,489		748,398
Ending balance: individually evaluated for impairment	902		915
Ending balance: collectively evaluated for impairment	736,587		747,483
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,371	7,522
Charge-offs	0	0
Recoveries	5	14
Provision	117	446
Valuation allowance, balance at the end of the period	6,493	7,982
Ending balance: individually evaluated for impairment	491		701
Ending balance: collectively evaluated for impairment	6,002		5,670
Financing Receivables:
Total financing receivables	995,624		978,237
Ending balance: individually evaluated for impairment	14,046		14,800
Ending balance: collectively evaluated for impairment	981,578		963,437
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	479	896
Charge-offs	0	(8)
Recoveries	7	5
Provision	(61)	(219)
Valuation allowance, balance at the end of the period	425	674
Ending balance: individually evaluated for impairment	4		6
Ending balance: collectively evaluated for impairment	421		473
Financing Receivables:
Total financing receivables	12,941		13,138
Ending balance: individually evaluated for impairment	2,065		2,059
Ending balance: collectively evaluated for impairment	10,876		11,079
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,790	4,671
Charge-offs	0	0
Recoveries	0	0
Provision	53	(310)
Valuation allowance, balance at the end of the period	2,843	4,361
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	2,843		2,790
Financing Receivables:
Total financing receivables	98,734		92,264
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	98,734		92,264
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4	12
Charge-offs	0	0
Recoveries	0	0
Provision	(1)	(8)
Valuation allowance, balance at the end of the period	3	4
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	3		4
Financing Receivables:
Total financing receivables	10,924		14,307
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	10,924		14,307
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	9,225	10,851
Charge-offs	(618)	(602)
Recoveries	180	1,170
Provision	2,027	(1,064)
Valuation allowance, balance at the end of the period	10,814	10,355
Ending balance: individually evaluated for impairment	2,965		628
Ending balance: collectively evaluated for impairment	7,849		8,597
Financing Receivables:
Total financing receivables	576,235		587,891
Ending balance: individually evaluated for impairment	15,895		5,340
Ending balance: collectively evaluated for impairment	560,340		582,551
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,769	10,987
Charge-offs	(5,559)	(4,232)
Recoveries	698	347
Provision	5,075	4,933
Valuation allowance, balance at the end of the period	16,983	$ 12,035
Ending balance: individually evaluated for impairment	4		4
Ending balance: collectively evaluated for impairment	16,979		16,765
Financing Receivables:
Total financing receivables	266,437		266,002
Ending balance: individually evaluated for impairment	88		89
Ending balance: collectively evaluated for impairment	$ 266,349		$ 265,913