Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 53,637	$ 55,533
Charge-offs	(4,873)	(4,340)
Recoveries	1,590	897
Provision	3,541	3,907
Valuation allowance, balance at the end of the period	53,895	55,997
Ending balance: individually evaluated for impairment	2,706		$ 2,730
Ending balance: collectively evaluated for impairment	51,189		50,907
Financing Receivables:
Total financing receivables	4,742,680		4,671,577
Ending balance: individually evaluated for impairment	34,631		33,393
Ending balance: collectively evaluated for impairment	4,708,049		4,638,184
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,902	2,873
Charge-offs	(31)	(6)
Recoveries	54	9
Provision	(400)	(95)
Valuation allowance, balance at the end of the period	2,525	2,781
Ending balance: individually evaluated for impairment	1,207		1,248
Ending balance: collectively evaluated for impairment	1,318		1,654
Financing Receivables:
Total financing receivables	2,116,121		2,118,047
Ending balance: individually evaluated for impairment	17,728		18,284
Ending balance: collectively evaluated for impairment	2,098,393		2,099,763
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	15,796	16,004
Charge-offs	0	0
Recoveries	0	0
Provision	163	500
Valuation allowance, balance at the end of the period	15,959	16,504
Ending balance: individually evaluated for impairment	68		65
Ending balance: collectively evaluated for impairment	15,891		15,731
Financing Receivables:
Total financing receivables	766,522		733,106
Ending balance: individually evaluated for impairment	1,004		1,016
Ending balance: collectively evaluated for impairment	765,518		732,090
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	7,522	5,039
Charge-offs	0	(14)
Recoveries	14	91
Provision	446	301
Valuation allowance, balance at the end of the period	7,982	5,417
Ending balance: individually evaluated for impairment	892		647
Ending balance: collectively evaluated for impairment	7,090		6,875
Financing Receivables:
Total financing receivables	914,941		913,052
Ending balance: individually evaluated for impairment	10,265		8,188
Ending balance: collectively evaluated for impairment	904,676		904,864
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	896	1,738
Charge-offs	(8)	0
Recoveries	5	203
Provision	(219)	(462)
Valuation allowance, balance at the end of the period	674	1,479
Ending balance: individually evaluated for impairment	17		47
Ending balance: collectively evaluated for impairment	657		849
Financing Receivables:
Total financing receivables	16,569		15,797
Ending balance: individually evaluated for impairment	1,184		1,265
Ending balance: collectively evaluated for impairment	15,385		14,532
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4,671	6,449
Charge-offs	0	0
Recoveries	0	0
Provision	(310)	808
Valuation allowance, balance at the end of the period	4,361	7,257
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	4,361		4,671
Financing Receivables:
Total financing receivables	114,535		108,273
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	114,535		108,273
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	12	12
Charge-offs	0	0
Recoveries	0	0
Provision	(8)	(1)
Valuation allowance, balance at the end of the period	4	11
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	4		12
Financing Receivables:
Total financing receivables	15,363		14,910
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	15,363		14,910
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	10,851	16,618
Charge-offs	(602)	(1,510)
Recoveries	1,170	297
Provision	(1,064)	(503)
Valuation allowance, balance at the end of the period	10,355	14,902
Ending balance: individually evaluated for impairment	519		694
Ending balance: collectively evaluated for impairment	9,836		10,157
Financing Receivables:
Total financing receivables	568,371		544,828
Ending balance: individually evaluated for impairment	4,385		4,574
Ending balance: collectively evaluated for impairment	563,986		540,254
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	10,987	6,800
Charge-offs	(4,232)	(2,810)
Recoveries	347	297
Provision	4,933	3,359
Valuation allowance, balance at the end of the period	12,035	7,646
Ending balance: individually evaluated for impairment	3		29
Ending balance: collectively evaluated for impairment	12,032		10,958
Financing Receivables:
Total financing receivables	230,258		223,564
Ending balance: individually evaluated for impairment	65		66
Ending balance: collectively evaluated for impairment	230,193		223,498
Unallocated
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	0	0
Charge-offs	0	0
Recoveries	0	0
Provision	0	0
Valuation allowance, balance at the end of the period	0	$ 0
Ending balance: collectively evaluated for impairment	$ 0		$ 0