Bank subsidiary (Details 7) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 41,004	$ 39,463	$ 41,985	$ 37,906
Charge-offs	(865)	(4,151)	(5,648)	(10,035)
Recoveries	859	918	3,762	2,435
Provision	54	3,580	953	9,504
Valuation allowance, balance at the end of the period	41,052	39,810	41,052	39,810
Valuation allowance, balance at the end of the period	41,052	39,810	41,052	39,810
Ending balance: individually evaluated for impairment	6,096	4,621	6,096	4,621
Ending balance: collectively evaluated for impairment	34,956	35,189	34,956	35,189
Financing Receivables:
Total financing receivables	4,055,389	3,757,903	4,055,389	3,757,903	3,790,856
Ending balance: individually evaluated for impairment	63,110	74,878	63,110	74,878
Ending balance: collectively evaluated for impairment	3,992,279	3,683,025	3,992,279	3,683,025
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,357	7,212	6,068	6,500
Charge-offs	(106)	(964)	(1,162)	(2,476)
Recoveries	157	379	1,382	974
Provision	(604)	100	(484)	1,729
Valuation allowance, balance at the end of the period	5,804	6,727	5,804	6,727
Valuation allowance, balance at the end of the period	5,804	6,727	5,804	6,727
Ending balance: individually evaluated for impairment	944	324	944	324
Ending balance: collectively evaluated for impairment	4,860	6,403	4,860	6,403
Financing Receivables:
Total financing receivables	2,015,082	1,899,580	2,015,082	1,899,580	1,866,450
Ending balance: individually evaluated for impairment	22,123	26,912	22,123	26,912
Ending balance: collectively evaluated for impairment	1,992,959	1,872,668	1,992,959	1,872,668
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	4,117	2,078	2,965	1,688
Provision	204	4	1,356	394
Valuation allowance, balance at the end of the period	4,321	2,082	4,321	2,082
Valuation allowance, balance at the end of the period	4,321	2,082	4,321	2,082
Ending balance: individually evaluated for impairment	888	7	888	7
Ending balance: collectively evaluated for impairment	3,433	2,075	3,433	2,075
Financing Receivables:
Total financing receivables	405,037	367,765	405,037	367,765	375,677
Ending balance: individually evaluated for impairment	4,484	2,929	4,484	2,929
Ending balance: collectively evaluated for impairment	400,553	364,836	400,553	364,836
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	5,009	4,843	4,493	4,354
Charge-offs	(44)	(363)	(782)	(402)
Recoveries	78	7	334	95
Provision	12	378	1,010	818
Valuation allowance, balance at the end of the period	5,055	4,865	5,055	4,865
Valuation allowance, balance at the end of the period	5,055	4,865	5,055	4,865
Ending balance: individually evaluated for impairment		313		313
Ending balance: collectively evaluated for impairment	5,055	4,552	5,055	4,552
Financing Receivables:
Total financing receivables	703,210	604,279	703,210	604,279	630,175
Ending balance: individually evaluated for impairment	960	1,913	960	1,913
Ending balance: collectively evaluated for impairment	702,250	602,366	702,250	602,366
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,187	3,340	4,275	3,795
Charge-offs	(3)	(1,093)	(238)	(2,340)
Recoveries	282	226	782	471
Provision	(361)	1,324	(2,714)	1,871
Valuation allowance, balance at the end of the period	2,105	3,797	2,105	3,797
Valuation allowance, balance at the end of the period	2,105	3,797	2,105	3,797
Ending balance: individually evaluated for impairment	1,585	2,321	1,585	2,321
Ending balance: collectively evaluated for impairment	520	1,476	520	1,476
Financing Receivables:
Total financing receivables	18,400	29,280	18,400	29,280	25,815
Ending balance: individually evaluated for impairment	12,747	25,146	12,747	25,146
Ending balance: collectively evaluated for impairment	5,653	4,134	5,653	4,134
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,305	2,255	2,023	1,888
Provision	44	(324)	326	43
Valuation allowance, balance at the end of the period	2,349	1,931	2,349	1,931
Valuation allowance, balance at the end of the period	2,349	1,931	2,349	1,931
Ending balance: collectively evaluated for impairment	2,349	1,931	2,349	1,931
Financing Receivables:
Total financing receivables	51,067	42,913	51,067	42,913	43,988
Ending balance: collectively evaluated for impairment	51,067	42,913	51,067	42,913
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	14	3	9	4
Provision	2	5	7	4
Valuation allowance, balance at the end of the period	16	8	16	8
Valuation allowance, balance at the end of the period	16	8	16	8
Ending balance: collectively evaluated for impairment	16	8	16	8
Financing Receivables:
Total financing receivables	10,460	5,648	10,460	5,648	6,171
Ending balance: collectively evaluated for impairment	10,460	5,648	10,460	5,648
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,307	13,961	15,931	14,867
Charge-offs	(305)	(1,130)	(1,655)	(2,964)
Recoveries	166	155	778	511
Provision	(361)	1,344	753	1,916
Valuation allowance, balance at the end of the period	15,807	14,330	15,807	14,330
Valuation allowance, balance at the end of the period	15,807	14,330	15,807	14,330
Ending balance: individually evaluated for impairment	2,679	1,656	2,679	1,656
Ending balance: collectively evaluated for impairment	13,128	12,674	13,128	12,674
Financing Receivables:
Total financing receivables	749,733	704,100	749,733	704,100	721,349
Ending balance: individually evaluated for impairment	22,777	17,956	22,777	17,956
Ending balance: collectively evaluated for impairment	726,956	686,144	726,956	686,144
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,399	3,797	4,019	3,806
Charge-offs	(407)	(601)	(1,811)	(1,853)
Recoveries	176	151	486	384
Provision	42	462	(484)	1,472
Valuation allowance, balance at the end of the period	2,210	3,809	2,210	3,809
Valuation allowance, balance at the end of the period	2,210	3,809	2,210	3,809
Ending balance: collectively evaluated for impairment	2,210	3,809	2,210	3,809
Financing Receivables:
Total financing receivables	102,400	104,338	102,400	104,338	121,231
Ending balance: individually evaluated for impairment	19	22	19	22
Ending balance: collectively evaluated for impairment	102,381	104,316	102,381	104,316
Unallocated
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,309	1,974	2,202	1,004
Provision	1,076	287	1,183	1,257
Valuation allowance, balance at the end of the period	3,385	2,261	3,385	2,261
Valuation allowance, balance at the end of the period	3,385	2,261	3,385	2,261
Ending balance: collectively evaluated for impairment	$ 3,385	$ 2,261	$ 3,385	$ 2,261