Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 54,297	$ 53,895	$ 52,119	$ 53,637
Charge-offs	(5,624)	(4,581)	(11,815)	(9,454)
Recoveries	2,064	726	3,563	2,316
Provision	7,688	2,763	14,558	6,304
Valuation allowance, balance at the end of the period	58,425	52,803	58,425	52,803
Ending balance: individually evaluated for impairment	6,860		6,860		$ 2,222
Ending balance: collectively evaluated for impairment	51,565		51,565		49,897
Financing Receivables:
Total financing receivables	5,008,145		5,008,145		4,843,634
Ending balance: individually evaluated for impairment	51,291		51,291		39,697
Ending balance: collectively evaluated for impairment	4,956,854		4,956,854		4,803,937
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	1,911	2,525	1,976	2,902
Charge-offs	(5)	0	(19)	(31)
Recoveries	8	14	617	68
Provision	101	400	(559)	0
Valuation allowance, balance at the end of the period	2,015	2,939	2,015	2,939
Ending balance: individually evaluated for impairment	904		904		876
Ending balance: collectively evaluated for impairment	1,111		1,111		1,100
Financing Receivables:
Total financing receivables	2,189,976		2,189,976		2,143,397
Ending balance: individually evaluated for impairment	17,537		17,537		16,494
Ending balance: collectively evaluated for impairment	2,172,439		2,172,439		2,126,903
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14,825	15,959	14,505	15,796
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	986	(661)	1,306	(498)
Valuation allowance, balance at the end of the period	15,811	15,298	15,811	15,298
Ending balance: individually evaluated for impairment	7		7		7
Ending balance: collectively evaluated for impairment	15,804		15,804		14,498
Financing Receivables:
Total financing receivables	790,174		790,174		748,398
Ending balance: individually evaluated for impairment	890		890		915
Ending balance: collectively evaluated for impairment	789,284		789,284		747,483
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,493	7,982	6,371	7,522
Charge-offs	(19)	(144)	(19)	(144)
Recoveries	4	13	9	27
Provision	403	(517)	520	(71)
Valuation allowance, balance at the end of the period	6,881	7,334	6,881	7,334
Ending balance: individually evaluated for impairment	465		465		701
Ending balance: collectively evaluated for impairment	6,416		6,416		5,670
Financing Receivables:
Total financing receivables	1,036,985		1,036,985		978,237
Ending balance: individually evaluated for impairment	13,376		13,376		14,800
Ending balance: collectively evaluated for impairment	1,023,609		1,023,609		963,437
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	425	674	479	896
Charge-offs	(4)	(9)	(4)	(17)
Recoveries	7	46	14	51
Provision	109	(69)	48	(288)
Valuation allowance, balance at the end of the period	537	642	537	642
Ending balance: individually evaluated for impairment	0		0		6
Ending balance: collectively evaluated for impairment	537		537		473
Financing Receivables:
Total financing receivables	14,696		14,696		13,138
Ending balance: individually evaluated for impairment	2,869		2,869		2,059
Ending balance: collectively evaluated for impairment	11,827		11,827		11,079
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,843	4,361	2,790	4,671
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(797)	255	(744)	(55)
Valuation allowance, balance at the end of the period	2,046	4,616	2,046	4,616
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	2,046		2,046		2,790
Financing Receivables:
Total financing receivables	68,676		68,676		92,264
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	68,676		68,676		92,264
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	3	4	4	12
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(1)	0	(2)	(8)
Valuation allowance, balance at the end of the period	2	4	2	4
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	2		2		4
Financing Receivables:
Total financing receivables	8,165		8,165		14,307
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	8,165		8,165		14,307
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	10,814	10,355	9,225	10,851
Charge-offs	(494)	(540)	(1,112)	(1,142)
Recoveries	1,281	280	1,461	1,450
Provision	1,472	66	3,499	(998)
Valuation allowance, balance at the end of the period	13,073	10,161	13,073	10,161
Ending balance: individually evaluated for impairment	4,983		4,983		628
Ending balance: collectively evaluated for impairment	8,090		8,090		8,597
Financing Receivables:
Total financing receivables	626,524		626,524		587,891
Ending balance: individually evaluated for impairment	16,033		16,033		5,340
Ending balance: collectively evaluated for impairment	610,491		610,491		582,551
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,983	12,035	16,769	10,987
Charge-offs	(5,102)	(3,888)	(10,661)	(8,120)
Recoveries	764	373	1,462	720
Provision	5,415	3,289	10,490	8,222
Valuation allowance, balance at the end of the period	18,060	$ 11,809	18,060	$ 11,809
Ending balance: individually evaluated for impairment	501		501		4
Ending balance: collectively evaluated for impairment	17,559		17,559		16,765
Financing Receivables:
Total financing receivables	272,949		272,949		266,002
Ending balance: individually evaluated for impairment	586		586		89
Ending balance: collectively evaluated for impairment	$ 272,363		$ 272,363		$ 265,913