Parent Company Only Financial Information, Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net income	$ 326,790	$ 333,562	$ 229,264
Adjustments to reconcile net income to net cash provided by operating activities:
Losses (gains) on sales of investment securities, net	(5,957)	1,985	(434)
Net change in other assets and other liabilities	(11,930)	(33,363)	(17,941)
Net cash provided by operating activities	574,260	496,567	458,368
Cash Flows from Investing Activities
Proceeds from sales of investment securities	1,367,476	601,130	18,467
Net cash provided by (used in) investing activities	1,617,446	(380,872)	(1,396,858)
Cash Flows from Financing Activities
Net increase (decrease) in commercial paper	308,039	(581,371)	(217,753)
Redemption of Corporation's senior notes	250,000	0	0
Proceeds from issuance of common stock for stock-based compensation plans	11,216	18,408	27,619
Proceeds from issuance of preferred stock	0	97,315	0
Purchase of preferred shares	0	(537)	0
Common stock warrants exercised		(1)
Purchase of common stock returned to authorized but unissued	0	(33,075)	(37,031)
Issuance of treasury stock for acquisition		488,408	7,151
Purchase of treasury stock	(186,076)	(213,598)	(9,290)
Cash dividends on common stock	(111,804)	(105,519)	(76,417)
Cash dividends on preferred stock	(15,202)	(10,784)	(9,347)
Net cash used in financing activities	(2,479,660)	44,983	1,012,275
Parent Company Only
Cash Flows from Operating Activities
Net income	326,790	333,562	229,264
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income (loss) of subsidiaries	(21,789)	(18,636)	(40,485)
Asset (gains) losses, net	0	0	(88)
Net change in other assets and other liabilities	265	(92,366)	(9,589)
Net cash provided by operating activities	305,266	222,562	179,102
Cash Flows from Investing Activities
Proceeds from sales of investment securities	0	827	2,618
Net (increase) decrease in notes receivable from subsidiaries	250,000	(139,317)	(300,000)
Net (increase) decrease in loans	0	2,210	1,058
Net cash provided by (used in) investing activities	250,000	(136,280)	(296,324)
Cash Flows from Financing Activities
Net increase (decrease) in commercial paper	(13,406)	(22,044)	(34,221)
Redemption of Corporation's senior notes	(250,000)	0	0
Proceeds from issuance of common stock for stock-based compensation plans	11,216	18,408	27,619
Proceeds from issuance of preferred stock	0	97,315	0
Purchase of preferred shares	0	(537)	0
Common stock warrants exercised	0	(1)	0
Purchase of common stock returned to authorized but unissued	0	(33,075)	(37,031)
Issuance of treasury stock for acquisition	0	91,296	0
Purchase of treasury stock	(186,076)	(213,598)	(9,290)
Cash dividends on common stock	(111,804)	(105,519)	(76,417)
Cash dividends on preferred stock	(15,202)	(10,784)	(9,347)
Net cash used in financing activities	(565,272)	(178,540)	(138,687)
Net increase (decrease) in cash and cash equivalents	(10,006)	(92,258)	(255,909)
Cash and cash equivalents at beginning of year	54,619	146,877	402,786
Cash and cash equivalents at end of year	$ 44,613	$ 54,619	$ 146,877