Parent Company Financial Statements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Overnight investments	$ 1,387,927,000	$ 1,872,594,000
Investment securities	7,180,180,000	7,006,580,000
Other assets	686,371,000	471,412,000
Total assets	34,527,512,000	32,990,836,000
Liabilities and Shareholders' Equity
Short-term borrowings	693,807,000	603,487,000
Long-term obligations	870,240,000	832,942,000
Other liabilities	261,784,000	283,629,000
Stockholders' equity	3,334,064,000	3,012,427,000	$ 2,872,109,000	$ 2,687,594,000
Total liabilities and shareholders' equity	34,527,512,000	32,990,836,000
Income Statement [Abstract]
Interest expense	43,794,000	43,082,000	44,304,000
Net interest income (loss)	1,059,896,000	944,675,000	924,905,000
Dividends from subsidiaries	50,424,000
Income tax benefit	219,946,000	125,585,000	122,028,000
Net income	323,752,000	225,482,000	210,386,000
OPERATING ACTIVITIES
Net income	323,752,000	225,482,000	210,386,000
Net amortization of premiums and accretion of discounts	(40,028,000)	(44,618,000)	(85,066,000)
Gain on elimination of acquired debt	(919,000)	(1,717,000)	0
Securities (gains) losses	(4,293,000)	(26,673,000)	(10,817,000)
Other than temporary impairment on securities	0
Change in other assets	(46,920,000)	(27,873,000)	(12,904,000)
Change in other liabilities	(29,542,000)	(25,520,000)	14,458,000
INVESTING ACTIVITIES
Net change in overnight investments	586,279,000	233,433,000	(338,213,000)
Business acquisitions, net of cash acquired	304,820,000	(727,000)	123,137,000
FINANCING ACTIVITIES
Net change in short-term borrowings	(44,680,000)	(33,072,000)	(397,952,000)
Retirement of long-term obligations	(6,955,000)	(9,279,000)	(5,896,000)
Cash dividends paid	(10,809,000)	(14,412,000)	(18,015,000)
Net change in cash	(203,591,000)	5,655,000	(70,096,000)
Cash and due from banks at beginning of period	539,741,000	534,086,000	604,182,000
Cash and due from banks at end of period	336,150,000	539,741,000	534,086,000
Cash payments for
Interest	43,639,000	44,998,000	46,785,000
Income taxes	88,565,000	108,741,000	136,900,000
Parent
Assets
Cash	45,411,000	8,278,000
Overnight investments	14,476,000	26,157,000
Investment securities	117,513,000	95,564,000
Investment in Banking Subsidiaries	3,203,491,000	2,932,048,000
Investment in Other Subsidiaries	41,165,000	41,066,000
Due from subsidiaries	4,000	0
Other assets	46,674,000	43,077,000
Total assets	3,468,734,000	3,146,190,000
Liabilities and Shareholders' Equity
Short-term borrowings	15,000,000	0
Long-term obligations	107,479,000	126,861,000
Due to Affiliate	728,000	2,350,000
Other liabilities	11,463,000	4,552,000
Stockholders' equity	3,334,064,000	3,012,427,000
Total liabilities and shareholders' equity	3,468,734,000	3,146,190,000
Income Statement [Abstract]
Interest income	921,000	1,110,000	645,000
Interest expense	4,814,000	6,067,000	6,793,000
Net interest income (loss)	(3,893,000)	(4,957,000)	(6,148,000)
Dividends from subsidiaries		90,055,000	75,000,000
Dividend Income, Banking Subsidiaries			75,006,000
Dividends from subsidiaries	0	0	23,500,000
Other income (loss)	8,377,000	9,330,000	1,870,000
Other operating expense	6,821,000	5,641,000	2,634,000
Income tax benefit	(5,395,000)	(730,000)	(2,618,000)
Income before income tax benefit and equity in undistributed net income of subsidiaries	48,087,000	88,787,000	91,594,000
Excess distributions (undistributed ) net income of subsidiaries	(270,270,000)	(135,965,000)	(116,174,000)
Net income	323,752,000	225,482,000	210,386,000
Income before equity in undistributed net income of subsidiaries	53,482,000	89,517,000	94,212,000
OPERATING ACTIVITIES
Net income	323,752,000	225,482,000	210,386,000
Excess distributions (undistributed ) net income of subsidiaries	(270,270,000)	(135,965,000)	(116,174,000)
Net amortization of premiums and accretion of discounts	759,000	398,000	(2,712,000)
Gain on elimination of acquired debt	(919,000)	(1,717,000)	0
Securities (gains) losses	(8,003,000)	(9,446,000)	(236,000)
Change in other assets	(10,509,000)	(980,000)	22,663,000
Change in other liabilities	2,707,000	2,483,000	(1,157,000)
Net cash provided by (Used in) Operating Activities	37,517,000	80,255,000	112,770,000
INVESTING ACTIVITIES
Net change in due from subsidiaries	(4,000)	0	299,889,000
Net change in overnight investments	11,681,000	(24,741,000)	(1,416,000)
Purchases of investment securities	(28,012,000)	(93,003,000)	(7,818,000)
Maturities and sales of investment securities	32,463,000	38,316,000	100,586,000
Net Cash Provided by (Used in) Investing Activities	16,128,000	(79,428,000)	391,241,000
FINANCING ACTIVITIES
Change in Due to Subsidiaries	(1,622,000)	2,296,000	54,000
Net change in short-term borrowings	0	0	(485,207,000)
Retirement of long-term obligations	(4,081,000)	(5,302,000)	0
Cash dividends paid	(10,809,000)	(14,412,000)	(18,015,000)
Net cash used by financing activities	(16,512,000)	(17,418,000)	(503,168,000)
Net change in cash	37,133,000	(16,591,000)	843,000
Cash and due from banks at beginning of period	8,278,000	24,869,000	24,026,000
Cash and due from banks at end of period	$ 45,411,000	$ 8,278,000	$ 24,869,000