Acquisitions - Additional Information (Details) $ / shares in Units, $ in Thousands	Jun. 17, 2019 Location City $ / shares	Jun. 14, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Total assets			$ 22,375,221	$ 22,693,402
Total deposits			16,887,629	$ 17,256,558
Closing price per share | $ / shares		$ 67.24
LegacyTexas [Member]
Business Acquisition [Line Items]
Total assets			9,900,000
Total gross loans			8,700,000
Total deposits			$ 7,100,000
LegacyTexas [Member]
Business Acquisition [Line Items]
Business acquisition, share consideration per share, percentage	52.80%
Business acquisition, cash for each share | $ / shares	$ 6.28
Total consideration		$ 2,100,000
Business acquisition, share price | $ / shares		$ 41.78
LegacyTexas [Member] | North Texas [Member]
Business Acquisition [Line Items]
Number of bank operating locations | Location	42
Number of bank operating cities | City	19