QUARTERLY FINANCIAL INFORMATION (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
QUARTERLY FINANCIAL INFORMATION (unaudited)
Interest and dividend income	$ 287,135	$ 285,948	$ 294,442	$ 286,173	$ 293,203	$ 281,706	$ 255,353	$ 238,423	$ 1,153,698	$ 1,068,685	$ 1,051,095
Interest expense	27,647	28,974	27,992	28,207	28,195	27,456	27,709	29,132	112,820	112,492	132,168
Net interest income	259,488	256,974	266,450	257,966	265,008	254,250	227,644	209,291	1,040,878	956,193	918,927
Provision for loan losses on non-covered loans	19,671	7,556	8,944	7,954	6,286	4,535	8,277	(762)	44,125	18,336	60,168
(Reversal of) provision for loan losses on covered loans	(671)	7,669	(944)	(1,021)	(820)	(964)	723	5,089	5,033	4,028	5,016
Net interest income after provision for loan losses	240,488	241,749	258,450	251,033	259,542	250,679	218,644	204,964	991,720	933,829	853,743
Noninterest (loss) income	7,805	10,342	(14,945)	(14,916)	(36,594)	(41,421)	(12,354)	(2,099)	(11,714)	(92,468)	(5,618)
Noninterest expense	135,246	176,979	127,899	124,427	124,384	100,352	94,420	96,355	564,551	415,511	422,533
Income before provision of income taxes	113,047	75,112	115,606	111,690	98,564	108,906	111,870	106,510	415,455	425,850	425,592
Provision (benefit) for income taxes	20,049	(13,644)	31,618	34,949	22,782	35,749	37,855	34,419	72,972	130,805	143,942
Net income	92,998	88,756	83,988	76,741	75,782	73,157	74,015	72,091	342,483	295,045	281,650
Preferred stock dividends							1,714	1,714		3,428	6,857
Net income available to common stockholders					$ 75,782	$ 73,157	$ 72,301	$ 70,377	$ 342,483	$ 291,617	$ 274,793
Basic earnings per share	$ 0.65	$ 0.62	$ 0.59	$ 0.54	$ 0.55	$ 0.53	$ 0.52	$ 0.51	$ 2.39	$ 2.11	$ 1.92
Diluted earnings per share	$ 0.65	$ 0.62	$ 0.58	$ 0.54	$ 0.55	$ 0.53	$ 0.52	$ 0.50	$ 2.38	$ 2.10	$ 1.89