Income taxes - Schedule of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Allowance for credit losses	$ 44,590	$ 48,409
Operating lease liabilities	14,247	14,496
Federal net operating loss	1,753	1,753
Deferred compensation	10,385	8,872
Unrealized loss on cash flow hedges	387	499
Other	18,804	19,101
Subtotal	90,166	93,130
Deferred tax liabilities:
FHLB stock dividends	(561)	(561)
Operating leases - right of use assets	(12,912)	(13,197)
Depreciation	(7,235)	(7,491)
Amortization of core deposit intangibles	(510)	(684)
Unrealized gain on equity securities	(3,785)	(17)
Unrealized gain on debt securities	(5,193)	(13,027)
Mortgage servicing rights	(27,107)	(20,803)
Goodwill	(11,912)	(11,301)
Other	(10,140)	(9,653)
Subtotal	(79,355)	(76,734)
Net deferred tax assets	$ 10,811	$ 16,396