Acquisition Activity - Schedule of Business Acquisitions by Acquisition, Equity Interest Issued or Issuable (Detail) - USD ($) $ in Thousands	May. 31, 2015	Mar. 31, 2015	Feb. 28, 2015	Sep. 30, 2015	Dec. 31, 2014	Dec. 31, 2013
Non-Equity consideration
Goodwill				$ 719,463	$ 517,526	$ 401,872
Florida Bank Group, Inc.
Equity consideration
Common stock issued (shares)			752,493
Total equity consideration			$ 47,497
Non-Equity consideration
Cash			42,988
Total consideration paid			90,485
Fair value of net assets assumed including identifiable intangible assets			73,736
Goodwill			$ 16,749
Old Florida Bancshares, Inc.
Equity consideration
Common stock issued (shares)		3,839,554
Total equity consideration		$ 242,007
Non-Equity consideration
Cash		11,145
Total consideration paid		253,152
Fair value of net assets assumed including identifiable intangible assets		153,621
Goodwill		$ 99,531
Georgia Commerce Bancshares, Inc.
Equity consideration
Common stock issued (shares)	2,882,357
Total equity consideration	$ 185,249
Non-Equity consideration
Cash	5,015
Total consideration paid	190,264
Fair value of net assets assumed including identifiable intangible assets	109,024
Goodwill	$ 81,240