Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 58,425	$ 52,803	$ 52,119	$ 53,637
Charge-offs	(10,231)	(5,377)	(22,046)	(14,831)
Recoveries	1,531	668	5,094	2,984
Provision	3,315	6,033	17,873	12,337
Valuation allowance, balance at the end of the period	53,040	54,127	53,040	54,127
Ending balance: individually evaluated for impairment	2,822		2,822		$ 2,222
Ending balance: collectively evaluated for impairment	50,218		50,218		49,897
Financing Receivables:
Total financing receivables	5,083,656		5,083,656		4,843,634
Ending balance: individually evaluated for impairment	43,464		43,464		39,697
Ending balance: collectively evaluated for impairment	5,040,192		5,040,192		4,803,937
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,015	2,939	1,976	2,902
Charge-offs	(7)	0	(26)	(31)
Recoveries	27	5	644	73
Provision	(56)	(623)	(615)	(623)
Valuation allowance, balance at the end of the period	1,979	2,321	1,979	2,321
Ending balance: individually evaluated for impairment	906		906		876
Ending balance: collectively evaluated for impairment	1,073		1,073		1,100
Financing Receivables:
Total financing receivables	2,183,888		2,183,888		2,143,397
Ending balance: individually evaluated for impairment	16,556		16,556		16,494
Ending balance: collectively evaluated for impairment	2,167,332		2,167,332		2,126,903
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	15,811	15,298	14,505	15,796
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(396)	(1,033)	910	(1,531)
Valuation allowance, balance at the end of the period	15,415	14,265	15,415	14,265
Ending balance: individually evaluated for impairment	7		7		7
Ending balance: collectively evaluated for impairment	15,408		15,408		14,498
Financing Receivables:
Total financing receivables	810,971		810,971		748,398
Ending balance: individually evaluated for impairment	877		877		915
Ending balance: collectively evaluated for impairment	810,094		810,094		747,483
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,881	7,334	6,371	7,522
Charge-offs	(13)	(80)	(32)	(224)
Recoveries	4	71	13	98
Provision	135	(347)	655	(418)
Valuation allowance, balance at the end of the period	7,007	6,978	7,007	6,978
Ending balance: individually evaluated for impairment	500		500		701
Ending balance: collectively evaluated for impairment	6,507		6,507		5,670
Financing Receivables:
Total financing receivables	1,079,262		1,079,262		978,237
Ending balance: individually evaluated for impairment	12,909		12,909		14,800
Ending balance: collectively evaluated for impairment	1,066,353		1,066,353		963,437
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	537	642	479	896
Charge-offs	0	(1)	(4)	(18)
Recoveries	28	122	42	173
Provision	(104)	(296)	(56)	(584)
Valuation allowance, balance at the end of the period	461	467	461	467
Ending balance: individually evaluated for impairment	0		0		6
Ending balance: collectively evaluated for impairment	461		461		473
Financing Receivables:
Total financing receivables	15,095		15,095		13,138
Ending balance: individually evaluated for impairment	3,194		3,194		2,059
Ending balance: collectively evaluated for impairment	11,901		11,901		11,079
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,046	4,616	2,790	4,671
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	196	(356)	(548)	(411)
Valuation allowance, balance at the end of the period	2,242	4,260	2,242	4,260
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	2,242		2,242		2,790
Financing Receivables:
Total financing receivables	76,382		76,382		92,264
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	76,382		76,382		92,264
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2	4	4	12
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	1	0	(1)	(8)
Valuation allowance, balance at the end of the period	3	4	3	4
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	3		3		4
Financing Receivables:
Total financing receivables	10,104		10,104		14,307
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	10,104		10,104		14,307
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	13,073	10,161	9,225	10,851
Charge-offs	(4,900)	(788)	(6,012)	(1,930)
Recoveries	726	105	2,187	1,555
Provision	(517)	1,255	2,982	257
Valuation allowance, balance at the end of the period	8,382	10,733	8,382	10,733
Ending balance: individually evaluated for impairment	905		905		628
Ending balance: collectively evaluated for impairment	7,477		7,477		8,597
Financing Receivables:
Total financing receivables	638,213		638,213		587,891
Ending balance: individually evaluated for impairment	9,370		9,370		5,340
Ending balance: collectively evaluated for impairment	628,843		628,843		582,551
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	18,060	11,809	16,769	10,987
Charge-offs	(5,311)	(4,508)	(15,972)	(12,628)
Recoveries	746	365	2,208	1,085
Provision	4,056	7,433	14,546	15,655
Valuation allowance, balance at the end of the period	17,551	$ 15,099	17,551	$ 15,099
Ending balance: individually evaluated for impairment	504		504		4
Ending balance: collectively evaluated for impairment	17,047		17,047		16,765
Financing Receivables:
Total financing receivables	269,741		269,741		266,002
Ending balance: individually evaluated for impairment	558		558		89
Ending balance: collectively evaluated for impairment	$ 269,183		$ 269,183		$ 265,913