Loans and Allowance for Loan Losses - Rollforward of Purchase Credit Impaired Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Gross Carrying Value
Gross carrying value, beginning balance	$ 12,468	$ 16,274
Acquisitions	80,812	1,359
Year-to-date settlements	18,956	5,165
Gross carrying value, ending balance	74,324	12,468
Accretable Yield
Accretable yield, beginning balance	0	0
Acquisitions	(196)	0
Year-to-date settlements	64	0
Accretable yield, ending balance	(132)	0
Nonaccretable Yield
Nonaccretable yield, beginning balance	(3,633)	(4,143)
Acquisitions	(32,314)	(812)
Year-to-date settlements	4,410	1,322
Nonaccretable yield, ending balance	(31,537)	(3,633)
Net Carrying Value
Net carrying value, beginning balance	8,835	12,131
Acquisitions	48,302	547
Year-to-date settlements	14,482	3,843
Net carrying value, ending balance	$ 42,655	$ 8,835