Parent Company Financial Statements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Overnight investments	$ 1,872,594,000	$ 2,063,132,000
Investment securities	7,006,580,000	6,861,293,000
Other assets	471,412,000	417,391,000
Total assets	32,990,836,000	31,475,934,000
Liabilities and Shareholders' Equity
Short-term borrowings	603,487,000	594,733,000
Long-term obligations	832,942,000	704,155,000
Other liabilities	283,629,000	247,729,000
Stockholders' equity	3,012,427,000	2,872,109,000	$ 2,687,594,000	$ 2,071,462,000
Total liabilities and shareholders' equity	32,990,836,000	31,475,934,000
Income Statement [Abstract]
Interest expense	43,082,000	44,304,000	50,351,000
Net interest income (loss)	944,675,000	924,905,000	710,097,000
Income tax benefit	125,585,000	122,028,000	65,032,000
Net income	225,482,000	210,386,000	138,562,000
OPERATING ACTIVITIES
Net income	225,482,000	210,386,000	138,562,000
Net amortization of premiums and accretion of discounts	(44,618,000)	(85,066,000)	(48,374,000)
Gain on elimination of acquired debt	0	0	(1,988,000)
Securities (gains) losses	(26,673,000)	(10,817,000)	(29,096,000)
Other than temporary impairment on securities	0
Change in other assets	(27,656,000)	(12,904,000)	(72,680,000)
Change in other liabilities	(25,520,000)	14,458,000	1,319,000
INVESTING ACTIVITIES
Net change in overnight investments	233,433,000	(338,213,000)	221,730,000
Business acquisitions, net of cash acquired	(727,000)	123,137,000	182,370,000
Net change in short-term borrowings	(33,072,000)	(397,952,000)	(25,321,000)
Retirement of long-term obligations	(11,213,000)	(5,896,000)	(54,301,000)
Stock issuance costs	0	0	(619,000)
Cash dividends paid	(14,412,000)	(18,015,000)	(11,543,000)
Net change in cash	5,655,000	(70,096,000)	70,583,000
Cash and due from banks at beginning of period	534,086,000	604,182,000	533,599,000
Cash and due from banks at end of period	539,741,000	534,086,000	604,182,000
Cash payments for
Interest	44,998,000	46,785,000	48,894,000
Income taxes	108,741,000	136,900,000	127,970,000
Parent
Assets
Cash	8,278,000	24,869,000
Overnight investments	26,157,000	1,416,000
Investment securities	95,564,000	21,137,000
Investment in Banking Subsidiaries	2,932,048,000	2,874,581,000
Investment in Other Subsidiaries	41,066,000	43,117,000
Other assets	92,787,000	73,944,000
Total assets	3,195,900,000	3,039,064,000
Liabilities and Shareholders' Equity
Long-term obligations	126,861,000	133,775,000
Due to Affiliate	56,323,000	29,682,000
Other liabilities	289,000	3,498,000
Stockholders' equity	3,012,427,000	2,872,109,000
Total liabilities and shareholders' equity	3,195,900,000	3,039,064,000
Income Statement [Abstract]
Interest income	1,110,000	645,000	1,784,000
Interest expense	6,067,000	6,793,000	9,694,000
Net interest income (loss)	(4,957,000)	(6,148,000)	(7,910,000)
Dividends from subsidiaries	90,055,000	75,006,000	30,000,000
Dividend Income, Banking Subsidiaries			82,419,000
Dividends from subsidiaries	0	23,500,000	0
Other income (loss)	9,330,000	1,870,000	33,600,000
Other operating expense	5,641,000	2,634,000	6,534,000
Income before income tax benefit and equity in undistributed net income of subsidiaries	88,787,000	91,594,000	101,575,000
Income tax benefit	(730,000)	(2,618,000)	(2,590,000)
Income before equity in undistributed net income of subsidiaries	89,517,000	94,212,000	104,165,000
Excess distributions (undistributed ) net income of subsidiaries	(135,965,000)	(116,174,000)	(34,397,000)
Net income	225,482,000	210,386,000	138,562,000
OPERATING ACTIVITIES
Net income	225,482,000	210,386,000	138,562,000
Excess distributions (undistributed ) net income of subsidiaries	(135,965,000)	(116,174,000)	(34,397,000)
Net amortization of premiums and accretion of discounts	(6,838,000)	(2,712,000)	594,000
Gain on elimination of acquired debt	0	0	(1,988,000)
Securities (gains) losses	(9,446,000)	(236,000)	(29,126,000)
Change in other assets	(20,845,000)	(3,070,000)	93,385,000
Change in other liabilities	(1,780,000)	(1,157,000)	2,250,000
Net cash provided by (Used in) Operating Activities	50,608,000	87,037,000	169,280,000
INVESTING ACTIVITIES
Net change in due from subsidiaries	0	295,994,000	(150,328,000)
Net change in overnight investments	(24,741,000)	(1,416,000)	0
Purchases of investment securities	(93,003,000)	(7,818,000)	(33,243,000)
Maturities and sales of investment securities	38,316,000	100,586,000	114,208,000
Investment in subsidiaries	0	0	1,579,000
Business acquisitions, net of cash acquired	0	0	(24,772,000)
Net Cash Provided by (Used in) Investing Activities	(79,428,000)	387,346,000	(92,556,000)
Change in Due to Subsidiaries	26,641,000	29,682,000	0
Net change in short-term borrowings	0	(485,207,000)	(1,211,000)
Retirement of long-term obligations	0	0	(52,372,000)
Stock issuance costs	0	0	(619,000)
Cash dividends paid	(14,412,000)	(18,015,000)	(11,543,000)
Net cash used by financing activities	12,229,000	(473,540,000)	(65,745,000)
Net change in cash	(16,591,000)	843,000	10,979,000
Cash and due from banks at beginning of period	24,869,000	24,026,000	13,047,000
Cash and due from banks at end of period	$ 8,278,000	$ 24,869,000	$ 24,026,000