Business Combinations - Additional Information (Details) $ in Thousands		3 Months Ended
	Jan. 01, 2018 USD ($) office branch	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets		$ 29,464,357	$ 24,002,306
Loans		22,419,905	18,210,724
Deposits		21,959,846	$ 18,153,462
USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Share ratio of common stock issued to USAB shareholders	6.1
Total consideration	$ 737,230
Merger expenses		$ 13,400
USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Assets	5,100,000
Loans	3,744,682
Deposits	$ 3,600,000
Number of offices | office	29
ALABAMA | USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Number of branches | branch	15