Acquisitions - Narrative (Details)		3 Months Ended		9 Months Ended
	Jul. 01, 2020 USD ($) bank_branch shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 369,018,000	$ 327,425,000	$ 369,018,000	$ 327,425,000	$ 327,425,000	$ 327,425,000	$ 327,425,000	$ 307,112,000
Merger-related costs excluded from pro forma		3,361,000	2,541,000	4,566,000	6,981,000
Three Shores
Business Acquisition [Line Items]
Number of banking offices | bank_branch	14
Assets acquired	$ 2,133,130,000
Liabilities assumed	1,987,026,000
Total consideration	187,697,000
Common stock issued in acquisition	163,589,000
Cash consideration	$ 24,108,000
Shares issued in acquisition (in shares) | shares	8,130,633
Goodwill	$ 41,593,000
Goodwill expected to be tax deductible	0
Core deposit intangible acquired	3,360,000
Three Shores | Merger-related costs
Business Acquisition [Line Items]
Merger-related costs excluded from pro forma		$ 3,360,000		$ 3,860,000
Three Shores | Core deposit intangible
Business Acquisition [Line Items]
Core deposit intangible acquired	$ 3,360,000
Expected useful life of acquired intangible (in years)	10 years
FMBT | Merger-related costs
Business Acquisition [Line Items]
Merger-related costs excluded from pro forma			$ 756,000		$ 1,780,000