Acquisition (Details)	Nov. 30, 2017 USD ($) banking_offices	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 201,116,000	$ 201,116,000
Business Acquisition, Goodwill, Expected Tax Deductible Amount		0
Diboll State Bancshares, Inc.
Business Acquisition [Line Items]
Voting interests acquired	100.00%
Number of branches | banking_offices	17
Acquisition, Total merger consideration	$ 224,300,000
Acquisition, Loans	621,300,000
Acquisition, Investment securities	234,400,000
Acquisition, Deposits	899,300,000
Goodwill		$ 109,600,000		$ 109,700,000
Core Deposits | Diboll State Bancshares, Inc.
Business Acquisition [Line Items]
Acquisition, Finite-lived intangible assets	$ 14,700,000
Amortization period	9 years
Trust Relationship Intangible | Diboll State Bancshares, Inc.
Business Acquisition [Line Items]
Acquisition, Finite-lived intangible assets	$ 5,400,000
Amortization period	13 years