Allowance for Loan Losses - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Minimum period interest and principal of loans past due considered nonperforming (in days)				90 days
Threshold for evaluation for impairment of substandard and nonaccrual commercial loans	$ 500			$ 500
Charge-offs				14,589	$ 13,496
Acquired loans from merger	5,949,019			5,949,019	5,765,934
Commercial and industrial customer
Financing Receivable, Allowance for Credit Losses [Line Items]
Charge-offs				8,574	4,709
Acquired loans from merger	1,493,416			1,493,416	1,433,266
Commercial and industrial customer | Automobiles and Equipment
Financing Receivable, Allowance for Credit Losses [Line Items]
Participating loans, total exposure			$ 4,900
Participating loans, direct exposure			950
Participating loans, recoveries		$ 100
Charge-offs			$ 5,200
Commercial construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Charge-offs	2,400			2,630	2,571
Commercial construction loans, residual loan exposure	11,500			11,500
Acquired loans from merger	257,197			257,197	384,334
Integrity Bancshares, Inc.
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans from merger	312,300			312,300	386,600
Integrity Bancshares, Inc. | Commercial and industrial customer
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans from merger	73,400			73,400	92,200
Integrity Bancshares, Inc. | Commercial construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans from merger	$ 6,600			6,600	$ 11,100
Commercial | Substandard
Financing Receivable, Allowance for Credit Losses [Line Items]
Increase in substandard loans resulted in loans downgraded				$ 110,500