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Acquisitions - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	May. 30, 2014
Business Acquisition [Line Items]
Goodwill	$ 654,668		$ 654,668		$ 654,668
Noninterest Expense	$ 173,239	$ 163,947	$ 344,750	$ 321,888
WNB Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Goodwill						$ 118,019
Purchase price allocation						198,798
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned						$ 149,700
Common stock number of shares issued for consideration				2,000,000
Amount of cash consideration				$ 49,100
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			10 years
Acquisition-related Costs [Member]
Business Acquisition [Line Items]
Noninterest Expense		$ 4,800		$ 6,000