Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Total aggregate amount of all noninterest expense.

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Acquisitions - Additional Information (Detail) (USD $)	12 Months Ended			9 Months Ended
Acquisitions - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	May 30, 2014
Business Acquisition [Line Items]
Non-interest expense	$ 654,740,000us-gaap_NoninterestExpense	$ 611,910,000us-gaap_NoninterestExpense	$ 575,093,000us-gaap_NoninterestExpense
WNB Bancshares, Inc. [Member]
Business Acquisition [Line Items]
Preliminary purchase price allocation					198,798,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned					149,700,000
Common stock number of shares issued for consideration	2,000,000
Amount of cash consideration	49,100,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				10 years
Acquisition-related Costs [Member]
Business Acquisition [Line Items]
Non-interest expense	$ 7,100,000	$ 1,400,000