Bank segment - Allowance for credit losses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	$ 53,355	$ 52,119
Charge-offs	(6,631)	(6,191)
Recoveries	1,018	1,499
Provision	9,901	6,870
Valuation allowance, balance at the end of the period	77,084	54,297
Ending balance: individually evaluated for impairment			$ 2,691
Ending balance: collectively evaluated for impairment			50,664
Financing Receivables:
Ending balance	5,180,378		5,120,664
Ending balance: individually evaluated for impairment			40,737
Ending balance: collectively evaluated for impairment			5,079,927
Residential 1-4 family
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	2,380	1,976
Charge-offs	0	(14)
Recoveries	53	609
Provision	(107)	(660)
Valuation allowance, balance at the end of the period	4,476	1,911
Ending balance: individually evaluated for impairment			898
Ending balance: collectively evaluated for impairment			1,482
Financing Receivables:
Ending balance			2,178,135
Ending balance: individually evaluated for impairment			15,600
Ending balance: collectively evaluated for impairment			2,162,535
Commercial real estate
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	15,053	14,505
Charge-offs	0	0
Recoveries	0	0
Provision	1,326	320
Valuation allowance, balance at the end of the period	16,587	14,825
Ending balance: individually evaluated for impairment			2
Ending balance: collectively evaluated for impairment			15,051
Financing Receivables:
Ending balance			824,830
Ending balance: individually evaluated for impairment			1,048
Ending balance: collectively evaluated for impairment			823,782
Home equity line of credit
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	6,922	6,371
Charge-offs	0	0
Recoveries	6	5
Provision	(162)	117
Valuation allowance, balance at the end of the period	6,225	6,493
Ending balance: individually evaluated for impairment			322
Ending balance: collectively evaluated for impairment			6,600
Financing Receivables:
Ending balance			1,092,125
Ending balance: individually evaluated for impairment			12,073
Ending balance: collectively evaluated for impairment			1,080,052
Residential land
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	449	479
Charge-offs	(8)	0
Recoveries	9	7
Provision	(34)	(61)
Valuation allowance, balance at the end of the period	352	425
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			449
Financing Receivables:
Ending balance			14,704
Ending balance: individually evaluated for impairment			3,091
Ending balance: collectively evaluated for impairment			11,613
Commercial construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	2,097	2,790
Charge-offs	0	0
Recoveries	0	0
Provision	1,060	53
Valuation allowance, balance at the end of the period	3,446	2,843
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			2,097
Financing Receivables:
Ending balance			70,605
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			70,605
Residential construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	3	4
Charge-offs	0	0
Recoveries	0	0
Provision	(3)	(1)
Valuation allowance, balance at the end of the period	14	3
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			3
Financing Receivables:
Ending balance			11,670
Ending balance: individually evaluated for impairment			0
Ending balance: collectively evaluated for impairment			11,670
Commercial loans
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	10,245	9,225
Charge-offs	(369)	(618)
Recoveries	186	180
Provision	1,993	2,027
Valuation allowance, balance at the end of the period	12,977	10,814
Ending balance: individually evaluated for impairment			1,015
Ending balance: collectively evaluated for impairment			9,230
Financing Receivables:
Ending balance			670,674
Ending balance: individually evaluated for impairment			8,418
Ending balance: collectively evaluated for impairment			662,256
Consumer loans
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	16,206	16,769
Charge-offs	(6,254)	(5,559)
Recoveries	764	698
Provision	5,828	5,075
Valuation allowance, balance at the end of the period	33,007	$ 16,983
Ending balance: individually evaluated for impairment			454
Ending balance: collectively evaluated for impairment			15,752
Financing Receivables:
Ending balance			257,921
Ending balance: individually evaluated for impairment			507
Ending balance: collectively evaluated for impairment			$ 257,414
Impact of ASU No. 2016-13
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	19,441
Impact of ASU No. 2016-13 | Residential 1-4 family
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	2,150
Impact of ASU No. 2016-13 | Commercial real estate
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	208
Impact of ASU No. 2016-13 | Home equity line of credit
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	(541)
Impact of ASU No. 2016-13 | Residential land
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	(64)
Impact of ASU No. 2016-13 | Commercial construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	289
Impact of ASU No. 2016-13 | Residential construction
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	14
Impact of ASU No. 2016-13 | Commercial loans
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	922
Impact of ASU No. 2016-13 | Consumer loans
Allowance for credit losses:
Valuation allowance, balance at the beginning of the period	$ 16,463