Parent Company Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Overnight investments		$ 4,347,336	$ 1,107,844
Investment in marketable equity securities		91,680	82,333
Investment securities available for sale		7,014,243	7,059,674
Other assets		783,953	610,891
Total assets		49,957,680	39,824,496
Liabilities and Shareholders' Equity
Short-term borrowings		641,487	738,233
Long-term obligations		1,248,163	588,638
Other liabilities		391,552	367,810
Stockholders' equity		4,229,268	3,586,184	$ 3,488,954	$ 3,334,064
Total liabilities and shareholders' equity		49,957,680	39,824,496
Income Statement [Abstract]
Interest expense		95,857	92,642	36,857
Net interest income		1,388,169	1,311,369	1,208,900
Dividends from subsidiaries		229,685
Income tax benefit	$ 13,900	126,159	134,677	103,297
Net income		491,723	457,371	400,313
Net income available to common shareholders		477,661	457,371	400,313
OPERATING ACTIVITIES
Net income		491,723	457,371	400,313
Net amortization of premiums and accretion of discounts		(8,513)	(27,263)	(32,291)
Marketable equity securities (gains) losses, net		(29,395)	(20,625)	7,610
Gain on elimination of acquired debt		0	0	(26,553)
Change in other assets		(7,286)	(24,274)	(5,076)
Change in other liabilities		(6,115)	(15,992)	9,105
Net cash provided by (Used in) Operating Activities		340,015	540,277	453,769
INVESTING ACTIVITIES
Net change in overnight investments		(3,204,363)	(65,181)	601,979
Purchases of marketable equity securities		(333,140)	(26,166)	(2,818)
Proceeds from sales of marketable equity securities		352,835	56,749	9,528
Investments in Majority-owned Subsidiaries		(422,500)	0	0
Net Cash Provided by (Used in) Investing Activities		(9,871,758)	(1,558,177)	101,140
FINANCING ACTIVITIES
Net change in short-term borrowings		(96,746)	(27,703)	(246,517)
Proceeds from Issuance of Long-term Debt		(400,000)	(200,000)	(125,000)
Repurchase of common stock		(333,755)	(453,123)	(163,095)
Cash dividends paid		(30,393)	(18,137)	(16,779)
Net cash used by financing activities		9,517,072	1,067,179	(563,619)
Cash payments for
Income taxes		116,583	83,038	73,806
Net proceeds from subordinated notes issuance		345,849	0	0
Net proceeds from preferred stock issuance		339,937	0	0
Parent
Assets
Cash		49,716	4,573
Overnight investments		1,607	2,547
Investment in marketable equity securities		91,680	82,333
Investment securities available for sale		2,010	3,015
Investment in Banking Subsidiaries		4,621,676	3,763,947
Investment in Other Subsidiaries		3,241	3,555
Due from subsidiaries		786	0
Other assets		48,591	45,164
Total assets		4,819,307	3,905,134
Liabilities and Shareholders' Equity
Short-term borrowings		452,350	105,677
Long-term obligations		128,125	201,702
Due to Affiliate		0	1,670
Other liabilities		9,564	9,901
Stockholders' equity		4,229,268	3,586,184
Total liabilities and shareholders' equity		4,819,307	3,905,134
Income Statement [Abstract]
Interest income		3,952	1,327	1,362
Interest expense		16,817	7,187	5,154
Net interest income		(12,865)	(5,860)	(3,792)
Dividends from subsidiaries			149,819	242,900
Dividend Income, Banking Subsidiaries				242,910
Marketable equity securities losses, net		29,395	20,625	(7,610)
Other income (loss)		574	257	347
Other operating expense		13,168	9,497	11,127
Income before income tax benefit and equity in undistributed net income of subsidiaries		233,621	155,344	220,728
Income tax benefit		879	892	(5,184)
Income before equity in undistributed net income of subsidiaries		232,742	154,452	225,912
Excess distributions (undistributed ) net income of subsidiaries		(258,981)	(302,919)	(174,401)
Net income		491,723	457,371	400,313
Less: Preferred stock dividends		14,062	0	0
Net income available to common shareholders		477,661	457,371	400,313
OPERATING ACTIVITIES
Net income		491,723	457,371	400,313
Excess distributions (undistributed ) net income of subsidiaries		(258,981)	(302,919)	(174,401)
Net amortization of premiums and accretion of discounts		824	119	88
Gain on elimination of acquired debt		0	0	(160)
Gain (Loss) on Disposition of Other Assets		2,456	2,185	381
Change in other assets		(3,074)	(2,001)	3,657
Securities (gains) losses		0	(20)	0
Change in other liabilities		(694)	981	(2,595)
Net cash provided by (Used in) Operating Activities		197,947	130,721	234,131
INVESTING ACTIVITIES
Net change in loans		0	100,000	(100,000)
Net change in overnight investments		940	2,162	14,091
Purchases of marketable equity securities		(333,140)	(26,166)	(2,818)
Proceeds from sales of marketable equity securities		352,835	56,749	9,528
Purchases of investment securities		0	0	(6,438)
Maturities and sales of investment securities		1,000	3,477	9,997
Net Cash Provided by (Used in) Investing Activities		(400,865)	136,222	(75,640)
FINANCING ACTIVITIES
Net change in short-term borrowings		(40,277)	40,277	(15,000)
Repayment of long-term obligations		(33,300)	(3,575)	(1,840)
Proceeds from Issuance of Long-term Debt		0	165,000	0
Repurchase of common stock		(333,755)	(453,123)	(163,095)
Cash dividends paid		(30,393)	(18,137)	(16,779)
Net cash used by financing activities		248,061	(269,558)	(196,714)
Net change in cash		45,143	(2,615)	(38,223)
Cash and due from banks at beginning of period		4,573	7,188	45,411
Cash and due from banks at end of period		49,716	4,573	7,188
Cash payments for
Interest		13,338	7,187	5,154
Income taxes		106,618	78,345	73,806
Net proceeds from subordinated notes issuance		345,849	0	0
Net proceeds from preferred stock issuance		$ 339,937	$ 0	$ 0