Bank segment - Allowance for loan losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	$ 55,533	$ 50,038
Charge-offs	(4,340)	(2,958)
Recoveries	897	480
Provision	3,907	4,766
Valuation allowance, balance at the end of the period	55,997	52,326
Ending balance: individually evaluated for impairment	3,700		$ 4,083
Ending balance: collectively evaluated for impairment	52,297		51,450
Financing Receivables:
Total financing receivables	4,729,792		4,743,619
Ending balance: individually evaluated for impairment	39,765		51,759
Ending balance: collectively evaluated for impairment	4,690,027		4,691,860
Residential 1-4 family
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	2,873	4,186
Charge-offs	(6)	(45)
Recoveries	9	17
Provision	(95)	435
Valuation allowance, balance at the end of the period	2,781	4,593
Ending balance: individually evaluated for impairment	1,386		1,352
Ending balance: collectively evaluated for impairment	1,395		1,521
Financing Receivables:
Total financing receivables	2,058,202		2,048,051
Ending balance: individually evaluated for impairment	19,340		19,854
Ending balance: collectively evaluated for impairment	2,038,862		2,028,197
Commercial real estate
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,004	11,342
Charge-offs	0	0
Recoveries	0	0
Provision	500	464
Valuation allowance, balance at the end of the period	16,504	11,806
Ending balance: individually evaluated for impairment	74		80
Ending balance: collectively evaluated for impairment	16,430		15,924
Financing Receivables:
Total financing receivables	790,191		800,395
Ending balance: individually evaluated for impairment	1,515		1,569
Ending balance: collectively evaluated for impairment	788,676		798,826
Home equity line of credit
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	5,039	7,260
Charge-offs	(14)	0
Recoveries	91	15
Provision	301	(103)
Valuation allowance, balance at the end of the period	5,417	7,172
Ending balance: individually evaluated for impairment	228		215
Ending balance: collectively evaluated for impairment	5,189		4,824
Financing Receivables:
Total financing receivables	866,880		863,163
Ending balance: individually evaluated for impairment	6,803		6,158
Ending balance: collectively evaluated for impairment	860,077		857,005
Residential land
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	1,738	1,671
Charge-offs	0	0
Recoveries	203	103
Provision	(462)	(34)
Valuation allowance, balance at the end of the period	1,479	1,740
Ending balance: individually evaluated for impairment	660		789
Ending balance: collectively evaluated for impairment	819		949
Financing Receivables:
Total financing receivables	16,888		18,889
Ending balance: individually evaluated for impairment	2,863		3,629
Ending balance: collectively evaluated for impairment	14,025		15,260
Commercial construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,449	4,461
Charge-offs	0	0
Recoveries	0	0
Provision	808	1,703
Valuation allowance, balance at the end of the period	7,257	6,164
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	7,257		6,449
Financing Receivables:
Total financing receivables	130,808		126,768
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	130,808		126,768
Residential construction
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	12	13
Charge-offs	0	0
Recoveries	0	0
Provision	(1)	(1)
Valuation allowance, balance at the end of the period	11	12
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	11		12
Financing Receivables:
Total financing receivables	13,694		16,080
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	13,694		16,080
Commercial loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	16,618	17,208
Charge-offs	(1,510)	(1,343)
Recoveries	297	135
Provision	(503)	991
Valuation allowance, balance at the end of the period	14,902	16,991
Ending balance: individually evaluated for impairment	1,318		1,641
Ending balance: collectively evaluated for impairment	13,584		14,977
Financing Receivables:
Total financing receivables	661,016		692,051
Ending balance: individually evaluated for impairment	9,175		20,539
Ending balance: collectively evaluated for impairment	651,841		671,512
Consumer loans
Allowance for loan losses:
Valuation allowance, balance at the beginning of the period	6,800	3,897
Charge-offs	(2,810)	(1,570)
Recoveries	297	210
Provision	3,359	1,311
Valuation allowance, balance at the end of the period	7,646	3,848
Ending balance: individually evaluated for impairment	34		6
Ending balance: collectively evaluated for impairment	7,612		6,794
Financing Receivables:
Total financing receivables	192,113		178,222
Ending balance: individually evaluated for impairment	69		10
Ending balance: collectively evaluated for impairment	192,044		178,212
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