Income taxes - Schedule of Net Deferred Tax liability (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses	$ 31,170		$ 8,113
Operating lease liability	8,924		9,373
Amortization of core deposit intangible	986		1,386
Deferred compensation	4,623		5,231
Unrealized loss on debt securities	54		54
Unrealized loss on equity securities	51		60
Unrealized loss on cash flow hedges	345		0
Other	3,041		2,388
Subtotal	49,194		26,605
Deferred tax liabilities:
FHLB stock dividends	(550)		(550)
Operating lease - right of use asset	(8,084)		(8,641)
Depreciation	(5,802)		(5,078)
Unrealized gain on cash flow hedges	0		(203)
Unrealized gain on debt securities	(7,758)		(3,051)
Mortgage servicing rights	(15,766)		(19,678)
Goodwill	(10,080)		(8,859)
Other	(1,234)		(1,035)
Subtotal	(49,274)		(47,095)
Net deferred tax liability	$ (80)	$ (20,490)	$ (20,490)