Bank segment - Allowance for loan commitments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2020
Allowance for loan commitments:
Beginning balance	$ 1,741	$ 8,100	$ 1,741	$ 1,741
Provision		(500)		4,300
Ending balance		7,600		$ 7,600
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Real estate | Home equity line of credit
Allowance for loan commitments:
Beginning balance	$ 392	300	$ 392	$ 392
Provision		0		0
Ending balance		300		300
Real estate | Commercial construction
Allowance for loan commitments:
Beginning balance	931	7,500	931	931
Provision		(800)		4,024
Ending balance		6,700		6,700
Commercial
Allowance for loan commitments:
Beginning balance	418	300	418	418
Provision		300		276
Ending balance		$ 600		600
Impact of ASU No. 2016-13
Allowance for loan commitments:
Beginning balance	1,559		1,559	1,559
Ending balance	2,000
Impact of ASU No. 2016-13 | Real estate | Home equity line of credit
Allowance for loan commitments:
Beginning balance	(92)		(92)	(92)
Impact of ASU No. 2016-13 | Real estate | Commercial construction
Allowance for loan commitments:
Beginning balance	1,745		1,745	1,745
Impact of ASU No. 2016-13 | Commercial
Allowance for loan commitments:
Beginning balance	$ (94)		$ (94)	$ (94)