Summary of significant accounting policies - Recent accounting standards (Details) $ in Millions		3 Months Ended
	May 06, 2020 USD ($) customer	Mar. 31, 2021 USD ($) customer loan	Dec. 31, 2020 USD ($) loan	Mar. 31, 2020 USD ($)
Maximum loan receivable deferred period	6 months
Number of business loans with payment deferrals granted \| customer		6
Loan receivable balances where payment deferrals were granted		$ (4.8)	$ 6.1
Outstanding loan balance in deferral		$ 1.3
SBA PPP
Number of loan forgiveness pending \| loan		468	1,304
Amount of loan forgiveness pending		$ 100.8	$ 189.7
Maximum
Percentage of total loan portfolio		0.10%
PPP
Percentage of total loan portfolio			0.30%
PPP \| SBA PPP
Number of loan applications \| loan			1,450
Amount of loans approved			$ 217.5
PPP \| Maximum
Percentage of total loan portfolio		0.10%
PPP Two \| SBA PPP
Number of loan applications \| loan		885
Amount of loans approved		$ 100.0
Commercial loans
Number of business loans with payment deferrals granted \| customer	481	6
Loan receivable balances where payment deferrals were granted	$ 306.9
Outstanding loan balance in deferral				$ 1.0
Consumer Loan
Number of business loans with payment deferrals granted \| customer	505	15
Loan receivable balances where payment deferrals were granted	$ 23.3
Outstanding loan balance in deferral				$ 0.3