Parent Company Only Financial Information, Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income	$ 229,264	$ 200,274	$ 188,301
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sales of investment securities, net	(434)	(9,316)	(8,133)
Asset (gains) losses, net	1,244	86	(2,540)
Net change in other assets and other liabilities	(27,497)	26,144	(11,685)
Net cash provided by operating activities	458,368	641,383	301,206
Cash Flows from Investing Activities
Proceeds from Sale of Available-for-sale Securities	18,467	549,555	1,601,947
Net cash used in investing activities	(1,396,858)	(1,653,023)	(1,533,818)
Cash Flows from Financing Activities
Net increase (decrease) in short-term funding	(415,753)	257,619	(233,872)
Repayment of long-term funding	115,020	1,180,038	1,500,035
Proceeds from issuance of common stock for stock-based compensation plans	(27,619)	(21,748)	(20,054)
Proceeds from issuance of preferred stock	0	97,066	62,966
Redemption of preferred stock	0	58,903	0
Purchase of preferred stock	0	(1,248)	(1,335)
Purchase of common stock returned to authorized but unissued	(37,031)	(20,007)	(93,000)
Purchase of treasury stock for tax withholding	(9,290)	(5,074)	(5,154)
Cash dividends on common stock	76,417	67,855	62,400
Cash dividends on preferred stock	9,347	8,903	7,155
Net cash provided by financing activities	1,012,275	1,180,188	674,230
Net increase (decrease) in cash and cash equivalents	73,785	168,548	(558,382)
Cash and cash equivalents at beginning of period	642,233	473,685	1,032,067
Cash and cash equivalents at end of period	716,018	642,233	473,685
Parent Company Only
Cash Flows from Operating Activities
Net income	229,264	200,274	188,301
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income (loss) of subsidiaries	(40,485)	(37,263)	(17,455)
Gain on sales of investment securities, net	0	(466)	0
Asset (gains) losses, net	(88)	(793)	(5,673)
Net change in other assets and other liabilities	(9,589)	19,708	(10,997)
Net cash provided by operating activities	179,102	181,460	154,176
Cash Flows from Investing Activities
Proceeds from Sale of Available-for-sale Securities	2,618	47,719	13,962
Increase (Decrease) in Notes Receivable, Related Parties	(300,000)	0	0
Proceeds from Sale of Productive Assets	1,058	2,211	11,964
Net cash used in investing activities	(296,324)	49,930	25,926
Cash Flows from Financing Activities
Net increase (decrease) in short-term funding	(34,221)	33,710	(6,319)
Repayment of long-term funding	0	(430,000)	0
Proceeds from issuance of common stock for stock-based compensation plans	27,619	21,748	20,054
Proceeds from issuance of preferred stock	0	97,066	62,966
Redemption of preferred stock	0	(58,903)	0
Purchase of preferred stock	0	(1,248)	(1,335)
Purchase of common stock returned to authorized but unissued	(37,031)	(20,007)	(93,000)
Purchase of treasury stock for tax withholding	(9,290)	(5,074)	(5,154)
Cash dividends on common stock	(76,417)	(67,855)	(62,400)
Cash dividends on preferred stock	(9,347)	(8,903)	(7,155)
Net cash provided by financing activities	(138,687)	(439,466)	(92,343)
Net increase (decrease) in cash and cash equivalents	(255,909)	(208,076)	87,759
Cash and cash equivalents at beginning of period	402,786	610,862	523,103
Cash and cash equivalents at end of period	$ 146,877	$ 402,786	$ 610,862