Business Combinations - Additional Information (Details) $ in Thousands		3 Months Ended	9 Months Ended
	Jan. 01, 2018 USD ($) office branch	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill from business combinations			$ 395,073
Assets		$ 30,881,948	30,881,948	$ 24,002,306
Loans		23,966,327	23,966,327	18,210,724
Deposits		22,588,272	$ 22,588,272	$ 18,153,462
Core Deposits
Business Acquisition [Line Items]
Estimated useful life			10 years
USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Assets	$ 5,100,000
Loans	3,700,000
Deposits	$ 3,600,000
Number of offices | office	29
USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Goodwill from business combinations		6,800
Loans	$ 3,735,162
Share ratio of common stock issued to USAB shareholders	6.1
Total consideration	$ 737,230
Merger expenses		$ 1,300	$ 18,100
USAmeriBancorp, Inc. | Core Deposits
Business Acquisition [Line Items]
Estimated useful life			10 years
USAmeriBancorp, Inc. | Alabama
Business Acquisition [Line Items]
Number of branches | branch	15