Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 45,795	$ 40,923	$ 45,618	$ 40,116
Charge-offs	(1,814)	(984)	(2,961)	(1,941)
Recoveries	559	1,412	1,269	2,181
Provision	1,825	1,021	2,439	2,016
Valuation allowance, balance at the end of the period	46,365	42,372	46,365	42,372
Ending balance: individually evaluated for impairment	5,390	4,370	5,390	4,370
Ending balance: collectively evaluated for impairment	40,975	38,002	40,975	38,002
Financing Receivables:
Total financing receivables	4,463,419	4,295,045	4,463,419	4,295,045	$ 4,440,989
Ending balance: individually evaluated for impairment	55,231	50,481	55,231	50,481
Ending balance: collectively evaluated for impairment	4,408,188	4,244,564	4,408,188	4,244,564
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4,921	5,475	4,662	5,534
Charge-offs	(58)	(94)	(214)	(360)
Recoveries	55	555	67	896
Provision	(627)	(269)	(224)	(403)
Valuation allowance, balance at the end of the period	4,291	5,667	4,291	5,667
Ending balance: individually evaluated for impairment	1,363	969	1,363	969
Ending balance: collectively evaluated for impairment	2,928	4,698	2,928	4,698
Financing Receivables:
Total financing receivables	2,045,357	2,019,092	2,045,357	2,019,092	2,044,205
Ending balance: individually evaluated for impairment	22,720	17,978	22,720	17,978
Ending balance: collectively evaluated for impairment	2,022,637	2,001,114	2,022,637	2,001,114
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	11,228	5,715	8,954	5,059
Provision	(808)	1,515	1,466	2,171
Valuation allowance, balance at the end of the period	10,420	7,230	10,420	7,230
Ending balance: individually evaluated for impairment	0	941	0	941
Ending balance: collectively evaluated for impairment	10,420	6,289	10,420	6,289
Financing Receivables:
Total financing receivables	561,262	476,116	561,262	476,116	531,917
Ending balance: individually evaluated for impairment	522	4,512	522	4,512
Ending balance: collectively evaluated for impairment	560,740	471,604	560,740	471,604
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,523	5,969	6,982	5,229
Charge-offs	(17)	(136)	(20)	(136)
Recoveries	8	314	39	325
Provision	99	934	(388)	1,663
Valuation allowance, balance at the end of the period	6,613	7,081	6,613	7,081
Ending balance: individually evaluated for impairment	269	14	269	14
Ending balance: collectively evaluated for impairment	6,344	7,067	6,344	7,067
Financing Receivables:
Total financing receivables	820,296	790,837	820,296	790,837	818,815
Ending balance: individually evaluated for impairment	1,899	612	1,899	612
Ending balance: collectively evaluated for impairment	818,397	790,225	818,397	790,225
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,286	1,575	1,875	1,817
Charge-offs	0	(47)	0	(53)
Recoveries	136	77	185	163
Provision	(319)	232	43	(90)
Valuation allowance, balance at the end of the period	2,103	1,837	2,103	1,837
Ending balance: individually evaluated for impairment	1,048	1,202	1,048	1,202
Ending balance: collectively evaluated for impairment	1,055	635	1,055	635
Financing Receivables:
Total financing receivables	17,273	17,189	17,273	17,189	16,240
Ending balance: individually evaluated for impairment	6,534	9,320	6,534	9,320
Ending balance: collectively evaluated for impairment	10,739	7,869	10,739	7,869
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,837	3,063	5,471	2,397
Provision	(262)	327	(2,896)	993
Valuation allowance, balance at the end of the period	2,575	3,390	2,575	3,390
Ending balance: collectively evaluated for impairment	2,575	3,390	2,575	3,390
Financing Receivables:
Total financing receivables	62,444	80,312	62,444	80,312	96,438
Ending balance: collectively evaluated for impairment	62,444	80,312	62,444	80,312
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	21	24	28	19
Provision	(3)	2	(10)	7
Valuation allowance, balance at the end of the period	18	26	18	26
Ending balance: collectively evaluated for impairment	18	26	18	26
Financing Receivables:
Total financing receivables	19,984	17,441	19,984	17,441	18,961
Ending balance: collectively evaluated for impairment	19,984	17,441	19,984	17,441
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14,580	15,592	14,017	15,803
Charge-offs	(756)	(246)	(802)	(370)
Recoveries	106	225	447	325
Provision	3,539	(427)	3,807	(614)
Valuation allowance, balance at the end of the period	17,469	15,144	17,469	15,144
Ending balance: individually evaluated for impairment	2,702	1,239	2,702	1,239
Ending balance: collectively evaluated for impairment	14,767	13,905	14,767	13,905
Financing Receivables:
Total financing receivables	821,636	782,804	821,636	782,804	791,757
Ending balance: individually evaluated for impairment	23,541	18,042	23,541	18,042
Ending balance: collectively evaluated for impairment	798,095	764,762	798,095	764,762
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	3,399	2,316	3,629	2,367
Charge-offs	(983)	(461)	(1,925)	(1,022)
Recoveries	254	241	531	472
Provision	206	(99)	641	180
Valuation allowance, balance at the end of the period	2,876	1,997	2,876	1,997
Ending balance: individually evaluated for impairment	8	5	8	5
Ending balance: collectively evaluated for impairment	2,868	1,992	2,868	1,992
Financing Receivables:
Total financing receivables	115,167	111,254	115,167	111,254	$ 122,656
Ending balance: individually evaluated for impairment	15	17	15	17
Ending balance: collectively evaluated for impairment	115,152	111,237	115,152	111,237
Unallocated
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	0	1,194	0	1,891
Provision	0	(1,194)	0	(1,891)
Valuation allowance, balance at the end of the period	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 0	$ 0	$ 0	$ 0