Parent Company Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Overnight investments	$ 797,406	$ 797,406	$ 1,387,927
Investment in marketable equity securities	92,599	92,599	0
Investment securities available for sale	4,557,110	4,557,110	7,180,180
Other assets	433,595	433,595	688,594
Total assets	35,408,629	35,408,629	34,527,512
Liabilities and Shareholders' Equity
Short-term borrowings	572,287	572,287	693,807
Long-term obligations	319,867	319,867	870,240
Other liabilities	249,443	249,443	261,784
Stockholders' equity	3,488,954	3,488,954	3,334,064	$ 3,012,427	$ 2,872,109
Total liabilities and shareholders' equity	35,408,629	35,408,629	34,527,512
Income Statement [Abstract]
Interest expense		36,857	43,794	43,082
Net interest income (loss)		1,208,900	1,059,896	944,675
Dividends from subsidiaries		242,910
Marketable equity securities losses, net	9	1,190
Income tax benefit		103,297	219,946	125,585
Net income		400,313	323,752	225,482
OPERATING ACTIVITIES
Net income		400,313	323,752	225,482
Net amortization of premiums and accretion of discounts		(36,567)	(40,028)	(44,618)
Change in the fair value of marketable equity securities		7,610	0	0
Gain on elimination of acquired debt		(26,553)	(12,483)	0
Securities (gains) losses		(351)	(4,293)	(26,673)
Change in other assets		304,503	(31,978)	(7,197)
Change in other liabilities		(40,895)	(25,939)	(25,520)
INVESTING ACTIVITIES
Net change in overnight investments		601,979	586,279	233,433
Purchases of marketable equity securities		(2,818)	0	0
Proceeds from sales of marketable equity securities		9,528	0	0
FINANCING ACTIVITIES
Repayment of long-term obligations		(752,447)	(6,955)	(9,279)
Repurchase of common stock		(163,095)	0	0
Cash dividends paid		(16,779)	(14,412)	(14,412)
Net change in cash		(8,710)	(203,591)	5,655
Cash and due from banks at beginning of period		336,150	539,741	534,086
Cash and due from banks at end of period	327,440	327,440	336,150	539,741
Cash payments for
Interest		37,097	43,639	44,998
Income taxes		73,806	88,565	108,741
Parent
Assets
Cash	7,188	7,188	45,411
Overnight investments	385	385	14,476
Investment in marketable equity securities	92,599	92,599	0
Investment securities available for sale	6,456	6,456	117,513
Investment in Banking Subsidiaries	3,314,292	3,314,292	3,203,491
Investment in Other Subsidiaries	41,830	41,830	41,165
Due from subsidiaries	814	814	4
Note to banking subsidiaries	100,000	100,000	0
Other assets	42,810	42,810	46,674
Total assets	3,606,374	3,606,374	3,468,734
Liabilities and Shareholders' Equity
Short-term borrowings	0	0	15,000
Long-term obligations	105,546	105,546	107,479
Due to Affiliate	299	299	728
Other liabilities	11,575	11,575	11,463
Stockholders' equity	3,488,954	3,488,954	3,334,064
Total liabilities and shareholders' equity	3,606,374	3,606,374	3,468,734
Income Statement [Abstract]
Interest income		1,362	921	1,110
Interest expense		5,154	4,814	6,067
Net interest income (loss)		(3,792)	(3,893)	(4,957)
Dividends from subsidiaries			50,424	90,100
Dividend Income, Banking Subsidiaries				90,055
Marketable equity securities losses, net		(7,610)	0	0
Other income (loss)		347	8,437	9,330
Other operating expense		11,127	6,881	5,641
Income tax benefit		(5,184)	(5,395)	(730)
Income before income tax benefit and equity in undistributed net income of subsidiaries		220,728	48,087	88,787
Excess distributions (undistributed ) net income of subsidiaries		(174,401)	(270,270)	(135,965)
Net income		400,313	323,752	225,482
Income before equity in undistributed net income of subsidiaries		225,912	53,482	89,517
OPERATING ACTIVITIES
Net income		400,313	323,752	225,482
Excess distributions (undistributed ) net income of subsidiaries		(174,401)	(270,270)	(135,965)
Net amortization of premiums and accretion of discounts		88	759	398
Gain on elimination of acquired debt		(160)	(919)	(1,717)
Securities (gains) losses		0	(8,003)	(9,446)
Change in other assets		3,657	(10,509)	(980)
Change in other liabilities		(2,595)	6,310	2,483
Net cash provided by (Used in) Operating Activities		234,512	41,120	80,255
INVESTING ACTIVITIES
Net change in loans		(100,000)	0	0
Net change in due from subsidiaries		(810)	(4)	0
Net change in overnight investments		14,091	11,681	(24,741)
Purchases of marketable equity securities		(2,818)	0	0
Proceeds from sales of marketable equity securities		9,528
Purchases of investment securities		(6,438)	(28,012)	(93,003)
Maturities and sales of investment securities		9,997	32,463	38,316
Net Cash Provided by (Used in) Investing Activities		(76,450)	16,128	(79,428)
FINANCING ACTIVITIES
Change in Due to Subsidiaries		429	(1,622)	2,296
Net change in short-term borrowings		(15,000)	0	0
Repayment of long-term obligations		(1,840)	(4,081)	(5,302)
Repurchase of common stock		(163,095)	0	0
Cash dividends paid		(16,779)	(14,412)	(14,412)
Net cash used by financing activities		(196,285)	(20,115)	(17,418)
Net change in cash		(38,223)	37,133	(16,591)
Cash and due from banks at beginning of period		45,411	8,278	24,869
Cash and due from banks at end of period	$ 7,188	7,188	45,411	8,278
Cash payments for
Interest		5,154	4,814	4,006
Income taxes		$ 73,806	$ 88,565	$ 108,741