Bank segment - Allowance for loan commitments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Allowance for loan commitments:
Beginning balance	$ 4,300	$ 1,741	$ 1,741
Provision	(1,400)	500
Ending balance	2,900	3,800		$ 1,741
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member
Real estate | Home equity line of credit
Allowance for loan commitments:
Beginning balance	300	392	$ 392
Provision	100	0
Ending balance	400	300		$ 392
Real estate | Commercial construction
Allowance for loan commitments:
Beginning balance	3,000	931	931
Provision	(1,700)	515
Ending balance	1,300	3,191		931
Commercial
Allowance for loan commitments:
Beginning balance	1,000	418	418
Provision	200	(15)
Ending balance	$ 1,200	309		418
Impact of adoption of ASU No. 2016-13
Allowance for loan commitments:
Beginning balance		1,559	1,559
Ending balance				1,559
Impact of adoption of ASU No. 2016-13 | Real estate | Home equity line of credit
Allowance for loan commitments:
Beginning balance		(92)	(92)
Ending balance				(92)
Impact of adoption of ASU No. 2016-13 | Real estate | Commercial construction
Allowance for loan commitments:
Beginning balance		1,745	1,745
Ending balance				1,745
Impact of adoption of ASU No. 2016-13 | Commercial
Allowance for loan commitments:
Beginning balance		$ (94)	$ (94)
Ending balance				$ (94)