Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 52,803	$ 56,356	$ 53,637	$ 55,533
Charge-offs	(5,377)	(4,897)	(14,831)	(12,471)
Recoveries	668	1,098	2,984	2,754
Provision	6,033	490	12,337	7,231
Valuation allowance, balance at the end of the period	54,127	53,047	54,127	53,047
Ending balance: individually evaluated for impairment	2,890		2,890		$ 2,730
Ending balance: collectively evaluated for impairment	51,237		51,237		50,907
Financing Receivables:
Total financing receivables	4,754,672		4,754,672		4,671,577
Ending balance: individually evaluated for impairment	41,160		41,160		33,393
Ending balance: collectively evaluated for impairment	4,713,512		4,713,512		4,638,184
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,939	3,130	2,902	2,873
Charge-offs	0	(522)	(31)	(528)
Recoveries	5	33	73	91
Provision	(623)	347	(623)	552
Valuation allowance, balance at the end of the period	2,321	2,988	2,321	2,988
Ending balance: individually evaluated for impairment	1,020		1,020		1,248
Ending balance: collectively evaluated for impairment	1,301		1,301		1,654
Financing Receivables:
Total financing receivables	2,110,489		2,110,489		2,118,047
Ending balance: individually evaluated for impairment	17,703		17,703		18,284
Ending balance: collectively evaluated for impairment	2,092,786		2,092,786		2,099,763
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	15,298	18,840	15,796	16,004
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(1,033)	(2,800)	(1,531)	36
Valuation allowance, balance at the end of the period	14,265	16,040	14,265	16,040
Ending balance: individually evaluated for impairment	51		51		65
Ending balance: collectively evaluated for impairment	14,214		14,214		15,731
Financing Receivables:
Total financing receivables	733,749		733,749		733,106
Ending balance: individually evaluated for impairment	981		981		1,016
Ending balance: collectively evaluated for impairment	732,768		732,768		732,090
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	7,334	5,527	7,522	5,039
Charge-offs	(80)	0	(224)	(14)
Recoveries	71	164	98	294
Provision	(347)	(36)	(418)	336
Valuation allowance, balance at the end of the period	6,978	5,655	6,978	5,655
Ending balance: individually evaluated for impairment	1,088		1,088		647
Ending balance: collectively evaluated for impairment	5,890		5,890		6,875
Financing Receivables:
Total financing receivables	949,872		949,872		913,052
Ending balance: individually evaluated for impairment	14,602		14,602		8,188
Ending balance: collectively evaluated for impairment	935,270		935,270		904,864
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	642	1,264	896	1,738
Charge-offs	(1)	0	(18)	(92)
Recoveries	122	259	173	477
Provision	(296)	(141)	(584)	(741)
Valuation allowance, balance at the end of the period	467	1,382	467	1,382
Ending balance: individually evaluated for impairment	0		0		47
Ending balance: collectively evaluated for impairment	467		467		849
Financing Receivables:
Total financing receivables	12,982		12,982		15,797
Ending balance: individually evaluated for impairment	2,057		2,057		1,265
Ending balance: collectively evaluated for impairment	10,925		10,925		14,532
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4,616	4,706	4,671	6,449
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	(356)	370	(411)	(1,373)
Valuation allowance, balance at the end of the period	4,260	5,076	4,260	5,076
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	4,260		4,260		4,671
Financing Receivables:
Total financing receivables	112,838		112,838		108,273
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	112,838		112,838		108,273
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4	9	12	12
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision	0	2	(8)	(1)
Valuation allowance, balance at the end of the period	4	11	4	11
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	4		4		12
Financing Receivables:
Total financing receivables	13,441		13,441		14,910
Ending balance: individually evaluated for impairment	0		0		0
Ending balance: collectively evaluated for impairment	13,441		13,441		14,910
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	10,161	14,552	10,851	16,618
Charge-offs	(788)	(1,215)	(1,930)	(3,477)
Recoveries	105	326	1,555	922
Provision	1,255	(595)	257	(995)
Valuation allowance, balance at the end of the period	10,733	13,068	10,733	13,068
Ending balance: individually evaluated for impairment	728		728		694
Ending balance: collectively evaluated for impairment	10,005		10,005		10,157
Financing Receivables:
Total financing receivables	574,243		574,243		544,828
Ending balance: individually evaluated for impairment	5,727		5,727		4,574
Ending balance: collectively evaluated for impairment	568,516		568,516		540,254
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	11,809	8,328	10,987	6,800
Charge-offs	(4,508)	(3,160)	(12,628)	(8,360)
Recoveries	365	316	1,085	970
Provision	7,433	3,343	15,655	9,417
Valuation allowance, balance at the end of the period	15,099	8,827	15,099	8,827
Ending balance: individually evaluated for impairment	3		3		29
Ending balance: collectively evaluated for impairment	15,096		15,096		10,958
Financing Receivables:
Total financing receivables	247,058		247,058		223,564
Ending balance: individually evaluated for impairment	90		90		66
Ending balance: collectively evaluated for impairment	246,968		246,968		223,498
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