Income taxes - Schedule of Net Deferred Tax Assets (Liability) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses	$ 51,869		$ 8,113
Operating lease liabilities	14,892		9,373
Federal net operating loss	1,753		0
Amortization of core deposit intangibles	0		1,386
Deferred compensation	8,250		5,231
Unrealized loss on debt securities	0		54
Unrealized loss on equity securities	41		60
Unrealized loss on cash flow hedges	347		0
Other	6,684		2,388
Subtotal	83,836		26,605
Deferred tax liabilities:
FHLB stock dividends	(561)		(550)
Operating leases - right of use assets	(13,863)		(8,641)
Depreciation	(5,689)		(5,078)
Amortization of core deposit intangibles	(668)		0
Unrealized gain on cash flow hedges	0		(203)
Unrealized gain on debt securities	(12,116)		(3,051)
Mortgage servicing rights	(18,557)		(19,678)
Goodwill	(10,691)		(8,859)
Other	200		(1,035)
Subtotal	(61,945)		(47,095)
Net deferred tax assets (liabilities)	21,891
Net deferred tax assets (liabilities)		$ (20,490)	(20,490)
Income Tax Contingency [Line Items]
Federal net operating loss	1,753		$ 0
Franklin Financial Network, Inc.
Deferred tax assets:
Federal net operating loss	8,346
Income Tax Contingency [Line Items]
Federal net operating loss	$ 8,346