Income taxes - Schedule of Net Deferred Tax liability (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses	$ 24,430		$ 8,113
Operating lease liability	9,125		9,373
Amortization of core deposit intangible	862		1,386
Deferred compensation	3,473		5,231
Unrealized loss on debt securities	54		54
Unrealized loss on equity securities	76		60
Unrealized loss on cash flow hedges	253		0
Other	2,677		2,388
Subtotal	40,950		26,605
Deferred tax liabilities:
FHLB stock dividends	(550)		(550)
Operating lease - right of use asset	(8,402)		(8,641)
Depreciation	(5,927)		(5,078)
Unrealized gain on cash flow hedges	0		(203)
Unrealized gain on debt securities	(7,490)		(3,051)
Mortgage servicing rights	(16,306)		(19,678)
Goodwill	(9,470)		(8,859)
Other	(1,000)		(1,035)
Subtotal	(49,145)		(47,095)
Net deferred tax liability	$ (8,195)	$ (20,490)	$ (20,490)