Business Combinations (Detail) $ in Thousands, shares in Millions				3 Months Ended								12 Months Ended
	Jan. 01, 2018 USD ($) office	Jan. 04, 2016 USD ($)	Dec. 01, 2015 USD ($) branch shares	Dec. 31, 2017 USD ($) branch	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) office branch	Dec. 31, 2016 USD ($) branch	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Charge within income tax expense				$ 34,958	$ 17,088	$ 20,714	$ 18,071	$ 18,336	$ 17,049	$ 15,460	$ 14,389	$ 90,831	$ 65,234	$ 23,938
Florida
Business Acquisition [Line Items]
Number of branches operated | branch				31								31
Number of branches closed | branch													5
USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Assets acquired				$ 4,700,000								$ 4,700,000
Loans acquired				3,800,000								3,800,000
Deposits acquired				$ 3,600,000								$ 3,600,000
Number of branches acquired | office												29
Merger expenses												$ 2,600
Masters Coverage Corp.
Business Acquisition [Line Items]
Total consideration for acquisition		$ 1,400
CNL Bancshares, Inc.
Business Acquisition [Line Items]
Assets acquired			$ 1,600,000
Loans acquired			825,000
Deposits acquired			$ 1,200,000
Conversion ratio for shares issued			0.705
Total consideration for acquisition			$ 230,000
Merger expenses														1,800
Shares issued in acquisition | shares			20.6
Charge within income tax expense														$ 3,300
CNL Bancshares, Inc. | Florida
Business Acquisition [Line Items]
Number of branches acquired | branch			16
Subsequent Event | USAmeriBancorp, Inc.
Business Acquisition [Line Items]
Conversion ratio for shares issued	6.1
Total consideration for acquisition	$ 737,000
Subsequent Event | USAmeriBancorp, Inc. | Alabama
Business Acquisition [Line Items]
Number of branches acquired | office	15