Parent Company Financial Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Overnight investments	$ 1,107,844	$ 797,406
Investment in marketable equity securities	82,333	92,599
Investment securities available for sale	7,059,674	4,557,110
Other assets	610,891	433,595
Total assets	39,824,496	35,408,629
Liabilities and Shareholders' Equity
Short-term borrowings	738,233	572,287
Long-term obligations	588,638	319,867
Other liabilities	367,810	249,443
Stockholders' equity	3,586,184	3,488,954	$ 3,334,064	$ 3,012,427
Total liabilities and shareholders' equity	39,824,496	35,408,629
Income Statement [Abstract]
Interest expense	92,642	36,857	43,794
Net interest income	1,311,369	1,208,900	1,059,896
Dividends from subsidiaries	149,819
Income tax benefit	134,677	103,297	219,946
Net income	457,371	400,313	323,752
OPERATING ACTIVITIES
Net income	457,371	400,313	323,752
Net amortization of premiums and accretion of discounts	(34,040)	(36,567)	(40,028)
Marketable equity securities (gains) losses, net	(20,625)	7,610	0
Gain on elimination of acquired debt	0	(26,553)	(919)
Change in other assets	(28,097)	(3,961)	(31,933)
Change in other liabilities	(19,584)	(40,895)	(25,939)
Net cash provided by (Used in) Operating Activities	540,569	457,336	355,258
INVESTING ACTIVITIES
Net change in overnight investments	(65,181)	601,979	586,279
Purchases of marketable equity securities	(26,166)	(2,818)	0
Proceeds from sales of marketable equity securities	56,749	9,528	0
Net Cash Provided by (Used in) Investing Activities	(1,558,469)	97,573	(668,672)
FINANCING ACTIVITIES
Net change in short-term borrowings	(27,703)	(246,517)	(44,680)
Proceeds from Issuance of Long-term Debt	(200,000)	(125,000)	(175,000)
Repurchase of common stock	(453,123)	(163,095)	0
Cash dividends paid	(18,137)	(16,779)	(14,412)
Net cash used by financing activities	1,067,179	(563,619)	109,823
Net change in cash	49,279	(8,710)	(203,591)
Cash and due from banks at beginning of period	327,440	336,150	539,741
Cash and due from banks at end of period	376,719	327,440	336,150
Cash payments for
Income taxes	83,038	73,806	88,565
Parent
Assets
Cash	4,573	7,188
Overnight investments	2,547	385
Investment in marketable equity securities	82,333	92,599
Investment securities available for sale	3,015	6,456
Investment in Banking Subsidiaries	3,763,947	3,314,292
Investment in Other Subsidiaries	3,555	41,830
Due from subsidiaries	0	814
Note to banking subsidiaries	0	100,000
Other assets	45,164	42,810
Total assets	3,905,134	3,606,374
Liabilities and Shareholders' Equity
Short-term borrowings	105,677	105,546
Long-term obligations	201,702	0
Due to Affiliate	1,670	299
Other liabilities	9,901	11,575
Stockholders' equity	3,586,184	3,488,954
Total liabilities and shareholders' equity	3,905,134	3,606,374
Income Statement [Abstract]
Interest income	1,327	1,362	921
Interest expense	7,187	5,154	4,814
Net interest income	(5,860)	(3,792)	(3,893)
Dividends from subsidiaries		242,910	50,400
Dividend Income, Banking Subsidiaries			50,424
Marketable equity securities losses, net	20,625	(7,610)	0
Other income (loss)	257	347	8,437
Other operating expense	9,497	11,127	6,881
Income tax benefit	892	(5,184)	(5,395)
Income before income tax benefit and equity in undistributed net income of subsidiaries	155,344	220,728	48,087
Excess distributions (undistributed ) net income of subsidiaries	(302,919)	(174,401)	(270,270)
Net income	457,371	400,313	323,752
Income before equity in undistributed net income of subsidiaries	154,452	225,912	53,482
OPERATING ACTIVITIES
Net income	457,371	400,313	323,752
Excess distributions (undistributed ) net income of subsidiaries	(302,919)	(174,401)	(270,270)
Net amortization of premiums and accretion of discounts	119	88	759
Gain on elimination of acquired debt	0	(160)	(919)
Gain (Loss) on Disposition of Other Assets	2,185	381	1,626
Change in other assets	(2,001)	3,657	(10,509)
Securities (gains) losses	(20)	0	(8,003)
Change in other liabilities	981	(2,595)	6,310
Net cash provided by (Used in) Operating Activities	130,721	234,131	39,494
INVESTING ACTIVITIES
Net change in loans	100,000	(100,000)	0
Net change in overnight investments	2,162	14,091	11,681
Purchases of marketable equity securities	(26,166)	(2,818)	0
Proceeds from sales of marketable equity securities	56,749	9,528	0
Purchases of investment securities	0	(6,438)	(28,012)
Maturities and sales of investment securities	3,477	9,997	32,463
Net Cash Provided by (Used in) Investing Activities	136,222	(75,640)	16,132
FINANCING ACTIVITIES
Net change in short-term borrowings	40,277	(15,000)	0
Repayment of long-term obligations	(3,575)	(1,840)	(4,081)
Proceeds from Issuance of Long-term Debt	165,000	0	0
Repurchase of common stock	(453,123)	(163,095)	0
Cash dividends paid	(18,137)	(16,779)	(14,412)
Net cash used by financing activities	(269,558)	(196,714)	(18,493)
Net change in cash	(2,615)	(38,223)	37,133
Cash and due from banks at beginning of period	7,188	45,411	8,278
Cash and due from banks at end of period	4,573	7,188	45,411
Cash payments for
Interest	7,187	5,154	4,814
Income taxes	$ 78,345	$ 73,806	$ 88,565