Goodwill and intangible assets - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 137,190	$ 137,190	$ 46,867
Amortization of core deposit and other intangibles		$ 3,185	$ 1,995	$ 2,132
Clayton Bank and Trust and American City Bank
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 90,323
Clayton Bank and Trust and American City Bank | Core deposit intangible
Finite-Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 9,060
Estimated useful life of intangible assets	3 years
Clayton Bank and Trust and American City Bank | Leasehold intangible
Finite-Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 587
Estimated useful life of intangible assets	6 years 6 months
Clayton Bank and Trust and American City Bank | Customer base trust intangible
Finite-Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 1,600
Estimated useful life of intangible assets	10 years
Clayton Bank and Trust and American City Bank | Manufactured housing servicing intangible
Finite-Lived Intangible Assets [Line Items]
Acquired intangible assets	$ 1,088
Estimated useful life of intangible assets	5 years