Parent Company Only Financial Information, Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities
Net income	$ 306,771	$ 326,790	$ 333,562
Adjustments to reconcile net income to net cash provided by operating activities:
Losses (gains) on sales of investment securities, net	(9,222)	(5,957)	1,985
Net change in other assets and other liabilities	32,270	(42,005)	(4,594)
Net cash provided by operating activities	550,020	574,260	496,567
Cash Flows from Investing Activities
Proceeds from sales of investment securities	626,283	1,367,476	601,130
Net cash provided by (used in) investing activities	(793,737)	1,617,446	(380,872)
Cash Flows from Financing Activities
Net increase (decrease) in commercial paper	(238,655)	308,039	(581,371)
Redemption of Corporation's senior notes	0	250,000	0
Proceeds from issuance of common stock for stock-based compensation plans	3,966	11,216	18,408
Proceeds from issuance of preferred stock	96,796	0	97,315
Purchase of preferred shares	0	0	(537)
Common stock warrants exercised			(1)
Purchase of common stock returned to authorized but unissued	0	0	(33,075)
Issuance of treasury stock for acquisition			488,408
Cash dividends on common stock	(112,023)	(111,804)	(105,519)
Cash dividends on preferred stock	(18,358)	(15,202)	(10,784)
Net cash used in financing activities	371,020	(2,479,660)	44,983
Performance/Service Based RSAs, RSA, [Member]
Cash Flows from Financing Activities
Payments for Repurchase of Common Stock	(6,113)	(8,592)	(7,148)
Parent Company Only
Cash Flows from Operating Activities
Net income	306,771	326,790	333,562
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income (loss) of subsidiaries	(61,406)	(21,789)	(18,636)
Net change in other assets and other liabilities	(49,890)	265	(92,366)
Net cash provided by operating activities	195,475	305,266	222,562
Cash Flows from Investing Activities
Proceeds from sales of investment securities	0	0	827
Net (increase) decrease in notes receivable from subsidiaries	(105,000)	250,000	(139,317)
Net (increase) decrease in loans	0	0	2,210
Net cash provided by (used in) investing activities	(105,000)	250,000	(136,280)
Cash Flows from Financing Activities
Net increase (decrease) in commercial paper	27,330	(13,406)	(22,044)
Redemption of Corporation's senior notes	0	(250,000)	0
Proceeds from issuance of common stock for stock-based compensation plans	3,966	11,216	18,408
Proceeds from issuance of preferred stock	96,796	0	97,315
Purchase of preferred shares	0	0	(537)
Common stock warrants exercised	0	0	(1)
Purchase of common stock returned to authorized but unissued	0	0	(33,075)
Issuance of treasury stock for acquisition	0	0	91,296
Purchase of treasury stock, open market purchases	(71,255)	(177,484)	(206,450)
Cash dividends on common stock	(112,023)	(111,804)	(105,519)
Cash dividends on preferred stock	(18,358)	(15,202)	(10,784)
Net cash used in financing activities	(79,656)	(565,272)	(178,540)
Net increase (decrease) in cash and cash equivalents	10,819	(10,006)	(92,258)
Cash and cash equivalents at beginning of year	44,613	54,619	146,877
Cash and cash equivalents at end of year	$ 55,432	$ 44,613	$ 54,619