Parent Company Financial Statements (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Investment securities	$ 7,171,917,000	$ 5,387,703,000
Other assets	394,027,000	258,568,000
Total assets	30,075,113,000	21,193,878,000
Liabilities and Shareholders' Equity
Short-term borrowings	987,184,000	511,418,000
Long-term obligations	351,320,000	510,769,000
Other liabilities	253,903,000	116,785,000
Stockholders' equity	2,687,594,000	2,071,462,000	1,859,624,000	1,861,128,000
Total liabilities and shareholders' equity	30,075,113,000	21,193,878,000
Income Statement [Abstract]
Interest expense	50,351,000	56,618,000	90,148,000
Net interest income (loss)	710,097,000	740,186,000	914,688,000
Income tax benefit	65,032,000	101,574,000	64,729,000
Net income	138,562,000	166,869,000	132,395,000
OPERATING ACTIVITIES
Net income	138,562,000	166,869,000	132,395,000
Net amortization of premiums and accretion of discounts	(48,374,000)	(112,759,000)	(156,796,000)
Gain on elimination of acquired debt	(1,988,000)	0	0
Securities (gains) losses	(29,096,000)	0	(2,277,000)
Gain on sale of other assets	0	(4,085,000)	0
Other than temporary impairment on securities	0
Change in other assets	(72,680,000)	100,437,000	(21,414,000)
Change in other liabilities	1,319,000	49,177,000	(77,590,000)
INVESTING ACTIVITIES
Net change in short-term borrowings	(25,321,000)	(57,087,000)	(101,717,000)
Retirement of long-term obligations	(54,301,000)	(4,152,000)	(196,338,000)
Stock issuance costs	(619,000)	0	0
Repurchase of common stock	0	(321,000)	(103,624,000)
Cash dividends paid	(11,543,000)	(8,663,000)	(15,398,000)
Net change in cash	70,583,000	(106,131,000)	48,929,000
Cash and due from banks at beginning of period	533,599,000	639,730,000	590,801,000
Cash and due from banks at end of period	604,182,000	533,599,000	639,730,000
Cash payments for
Interest	48,894,000	59,234,000	104,514,000
Income taxes	127,970,000	102,890,000	66,453,000
Parent
Assets
Cash	24,026,000	13,047,000
Investment securities	110,644,000	234,488,000
Investment in subsidiaries	2,815,866,000	2,053,292,000
Due from subsidiaries	295,994,000	145,666,000
Other assets	74,157,000	144,998,000
Total assets	3,320,687,000	2,591,491,000
Liabilities and Shareholders' Equity
Short-term borrowings	485,207,000	411,907,000
Long-term obligations	136,717,000	96,392,000
Other liabilities	11,169,000	11,730,000
Stockholders' equity	2,687,594,000	2,071,462,000
Total liabilities and shareholders' equity	3,320,687,000	2,591,491,000
Income Statement [Abstract]
Interest income	1,784,000	1,387,000	1,353,000
Interest expense	9,694,000	7,065,000	15,435,000
Net interest income (loss)	(7,910,000)	(5,678,000)	(14,082,000)
Dividends from subsidiaries	82,419,000	131,006,000	179,588,000
Other income (loss)	33,600,000	3,620,000	2,843,000
Other operating expense	6,534,000	2,344,000	6,384,000
Income before income tax benefit and equity in undistributed net income of subsidiaries	101,575,000	126,604,000	161,965,000
Income tax benefit	(2,590,000)	(2,095,000)	(8,417,000)
Income before equity in undistributed net income of subsidiaries	104,165,000	128,699,000	170,382,000
Excess distributions (undistributed ) net income of subsidiaries	(34,397,000)	(38,170,000)	37,987,000
Net income	138,562,000	166,869,000	132,395,000
OPERATING ACTIVITIES
Net income	138,562,000	166,869,000	132,395,000
Excess distributions (undistributed ) net income of subsidiaries	(34,397,000)	(38,170,000)	37,987,000
Net amortization of premiums and accretion of discounts	594,000	334,000	439,000
Gain on elimination of acquired debt	(1,988,000)	0	0
Securities (gains) losses	(29,126,000)	0	(2,274,000)
Gain on sale of other assets	0	(1,331,000)	0
Other than temporary impairment on securities	0	0	45,000
Change in other assets	93,385,000	(61,704,000)	30,761,000
Change in other liabilities	2,250,000	(2,096,000)	(10,148,000)
Net cash provided by (Used in) Operating Activities	169,280,000	63,902,000	189,205,000
INVESTING ACTIVITIES
Net change in due from subsidiaries	(150,328,000)	(67,154,000)	42,323,000
Purchases of investment securities	(33,243,000)	(126,197,000)	(111,409,000)
Maturities and sales of investment securities	114,208,000	135,000,000	112,625,000
Investment in subsidiaries	1,579,000	1,489,000	9,298,000
Business acquisitions, net of cash acquired	(24,772,000)	0	0
Net Cash Provided by (Used in) Investing Activities	(92,556,000)	(56,862,000)	52,837,000
Net change in short-term borrowings	(1,211,000)	12,860,000	23,651,000
Retirement of long-term obligations	(52,372,000)	0	(155,305,000)
Stock issuance costs	(619,000)	0	0
Repurchase of common stock	0	(321,000)	(103,624,000)
Cash dividends paid	(11,543,000)	(8,663,000)	(15,398,000)
Net cash used by financing activities	(65,745,000)	3,876,000	(250,676,000)
Net change in cash	10,979,000	10,916,000	(8,634,000)
Cash and due from banks at beginning of period	13,047,000	2,131,000	10,765,000
Cash and due from banks at end of period	24,026,000	13,047,000	2,131,000
Cash payments for
Interest	5,079,000	6,904,000	25,574,000
Income taxes	$ 127,970,000	$ 102,890,000	$ 66,453,000