Parent Company Information (Components of Condensed Cash Flow The Parent Company) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation	$ 12,276	$ 11,438	$ 10,935
Gain on redemption of other assets	0	(7,331)	0
Net cash provided by operating activities	444,966	398,145	302,792
Investing activities:
Purchases of available-for-sale securities	(660,106)	(980,870)	(903,240)
Proceeds from sales of available-for-sale securities	0	259,283	123,270
Purchases of intercompany debt securities	873	(381)	458
Net cash provided by (used for) investing activities	(527,220)	(1,635,006)	(774,184)
Financing activities:
Proceeds from Issuance of Redeemable Preferred Stock	145,056	0	0
Payments for Repurchase of Redeemable Preferred Stock	(122,710)	0	0
Cash dividends paid to common shareholders	(94,630)	(89,522)	(80,964)
Cash dividends paid to preferred shareholders	(8,096)	(8,096)	(8,711)
Exercise of stock options	8,259	11,762	3,060
Excess tax benefits from stock-based compensation	0	3,204	2,338
Common stock repurchased/shares acquired related to employee share-based plans	(11,585)	(11,206)	(12,564)
Common stock warrants repurchased	0	(163)	(23)
Net cash (used for) provided by financing activities	122,749	1,227,831	457,171
Net decrease in cash and due from banks	40,495	(9,030)	(14,221)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	190,663	199,693	213,914
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	231,158	190,663	199,693
Parent Company [Member]
Operating activities:
Net income	255,439	207,127	204,729
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed earnings of subsidiaries and associated companies	(159,638)	(79,599)	(115,463)
Stock-based compensation	12,276	11,438	10,935
Gain on redemption of other assets	0	(7,331)	0
Other, net	7,880	(3,736)	9,066
Net cash provided by operating activities	115,957	127,899	109,267
Investing activities:
Proceeds from sales of available-for-sale securities	0	1,089	0
Purchases of intercompany debt securities	0	(150,000)	0
Proceeds from Sale of Other Assets, Investing Activities	7,581	0	0
Net cash provided by (used for) investing activities	7,581	(148,911)	0
Financing activities:
Proceeds from Issuance of Redeemable Preferred Stock	145,056	0	0
Payments for Repurchase of Redeemable Preferred Stock	(122,710)	0	0
Cash dividends paid to common shareholders	(94,630)	(89,522)	(80,964)
Cash dividends paid to preferred shareholders	(8,096)	(8,096)	(8,711)
Exercise of stock options	8,259	11,762	3,060
Excess tax benefits from stock-based compensation	0	3,204	2,338
Common stock repurchased/shares acquired related to employee share-based plans	(23,279)	(22,870)	(17,815)
Common stock warrants repurchased	0	(163)	(23)
Net cash (used for) provided by financing activities	(95,400)	(105,685)	(102,115)
Net decrease in cash and due from banks	28,138	(126,697)	7,152
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Beginning Balance	152,947	279,644	272,492
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Ending Balance	$ 181,085	$ 152,947	$ 279,644