Loans and Allowance for Loan Losses - Changes in Value of Accretable Yield for PCI Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Notes And Loans Receivable [Line Items]
Purchased Credit Impaired Accretable yield, Accretion	$ 5,419	$ 3,538	$ 493
Purchased Credit Impaired
Accounts Notes And Loans Receivable [Line Items]
Purchased Credit Impaired Accretable yield, Beginning balance	(2,444)	(1,637)
Purchased Credit Impaired Accretable yield, Additions through the acquisition of the Clayton Banks	(18,868)		(1,991)
Purchased Credit Impaired Accretable Yield, Principal reductions and other reclassifications from nonaccretable difference	(1,841)	(3,438)	(100)
Purchased Credit Impaired Accretable yield, Recoveries	(23)
Purchased Credit Impaired Accretable yield, Accretion	5,299	2,631	454
Purchased Credit Impaired Accretable yield, Other changes	195
Purchased Credit Impaired Accretable yield, Ending balance	$ (17,682)	$ (2,444)	$ (1,637)