Acquisition - Narrative (Details) $ in Thousands		3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 01, 2019 USD ($) bank_branch shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Less: Common stock issued		$ 229,926	$ 229,926	$ 0
Common Stock
Business Acquisition [Line Items]
Stock issued as a part of acquisition (in shares) | shares		6,383,806	6,383,806
Less: Common stock issued		$ 798	$ 798
MBT
Business Acquisition [Line Items]
Percentage of interest acquired	100.00%
Number of banking centers acquired | bank_branch	20
Less: Common stock issued	$ 229,900
Other intangible assets	16,527
Operating revenue		4,900	4,900
Non-recurring expenses		$ 17,300	$ 18,200		$ 877
MBT | Core deposit intangibles acquired
Business Acquisition [Line Items]
Other intangible assets	$ 16,527
Acquired intangible asset, expected useful life	10 years
MBT | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock	0.275
Stock issued as a part of acquisition (in shares) | shares	6,400,000