Acquisitions - Textual (Details)			9 Months Ended
	Jul. 30, 2019 $ / shares shares	Apr. 12, 2019 USD ($) shares	Sep. 30, 2019 USD ($) financial_center $ / shares	Oct. 29, 2019 shares	Jun. 30, 2019 USD ($)	Apr. 11, 2019 financial_center	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Stock issued for Reliance acquisition			$ 144,830,000
Number of financial centers | financial_center			212
Assets			$ 17,758,511,000				$ 16,543,337,000	$ 15,000,000,000
Net loans			12,936,959,000				11,666,572,000
Deposits			13,468,009,000				12,398,752,000
Goodwill			$ 926,648,000				$ 845,687,000
Common stock, Class A, par value (in dollars per share) | $ / shares			$ 0.01				$ 0.01
Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Shares issued (in shares) | shares		3,999,623
Payments to acquire businesses, gross		$ 62,700,000
Stock issued for Reliance acquisition		42,000,000.0
Number of financial centers | financial_center						22
Assets		1,500,000,000
Net loans		1,100,000,000
Deposits		1,200,000,000
Goodwill		$ 80,961,000			$ 81,000,000.0
Landrum Company
Business Acquisition [Line Items]
Shares issued (in shares) | shares	17,350,000
Number of financial centers | financial_center			39
Assets			$ 3,300,000,000
Net loans			2,000,000,000.0
Deposits			$ 2,900,000,000
Series A or B Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Shares issued, conversion ratio (in shares)		1
Series C Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Shares issued, conversion ratio (in shares)		1
Common Class A | Landrum Company
Business Acquisition [Line Items]
Common stock, Class A, par value (in dollars per share) | $ / shares	$ 0.01
Preferred Class B | Landrum Company
Business Acquisition [Line Items]
Preferred stock, Class B, par value (in dollars per share) | $ / shares	$ 0.01
Subsequent Event | Series C Preferred Stock | Reliance Bancshares, Inc.
Business Acquisition [Line Items]
Preferred stock, shares authorized (in shares) | shares				140
Preferred stock, shares issued (in shares) | shares				0
Preferred stock, shares outstanding (in shares) | shares				0