Parent Company Only Financial Information, Statement of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities
Net income		$ 333,562	$ 229,264	$ 200,274
Adjustments to reconcile net income to net cash provided by operating activities:
Losses (gains) on sales of investment securities, net		1,985	(434)	(9,316)
Asset (gains) losses, net		1,103	1,244	86
Net change in other assets and other liabilities		(2,763)	(21,919)	26,144
Net cash provided by operating activities		496,567	458,368	641,383
Cash Flows from Investing Activities
Proceeds from Sale of Available-for-sale Securities	$ 108,000	601,130	18,467	549,555
Net cash used in investing activities		(380,872)	(1,396,858)	(1,653,023)
Cash Flows from Financing Activities
Net increase (decrease) in short-term funding		(581,371)	(217,753)	110,619
Repayment of long-term funding		0	0	430,000
Proceeds from issuance of common stock for stock-based compensation plans		18,408	27,619	21,748
Proceeds from issuance of preferred stock		97,315	0	97,066
Redemption of preferred stock		0	0	58,903
Purchase of preferred shares		(537)	0	(1,248)
Common stock warrants exercised		(1)
Purchase of common stock returned to authorized but unissued		(33,075)	(37,031)	(20,007)
Issuance of Treasury Stock for Acquisition		488,408	7,151
Purchase of treasury stock		(213,598)	(9,290)	(5,074)
Cash dividends on common stock		(105,519)	(76,417)	(67,855)
Cash dividends on preferred stock		(10,784)	(9,347)	(8,903)
Net cash used in financing activities		44,983	1,012,275	1,180,188
Parent Company Only
Cash Flows from Operating Activities
Net income		333,562	229,264	200,274
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income (loss) of subsidiaries		(18,636)	(40,485)	(37,263)
Losses (gains) on sales of investment securities, net		0	0	(466)
Asset (gains) losses, net		0	(88)	(793)
Net change in other assets and other liabilities		(92,366)	(9,589)	19,708
Net cash provided by operating activities		222,562	179,102	181,460
Cash Flows from Investing Activities
Proceeds from Sale of Available-for-sale Securities		827	2,618	47,719
Net increase in notes receivable from subsidiaries		(139,317)	(300,000)	0
Purchase of other assets, net of disposals		2,210	1,058	2,211
Net cash used in investing activities		(136,280)	(296,324)	49,930
Cash Flows from Financing Activities
Net increase (decrease) in short-term funding		(22,044)	(34,221)	33,710
Repayment of long-term funding		0	0	(430,000)
Proceeds from issuance of common stock for stock-based compensation plans		18,408	27,619	21,748
Proceeds from issuance of preferred stock		97,315	0	97,066
Redemption of preferred stock		0	0	(58,903)
Purchase of preferred shares		(537)	0	(1,248)
Common stock warrants exercised		(1)	0	0
Purchase of common stock returned to authorized but unissued		(33,075)	(37,031)	(20,007)
Issuance of Treasury Stock for Acquisition		91,296	0	0
Purchase of treasury stock		(213,598)	(9,290)	(5,074)
Cash dividends on common stock		(105,519)	(76,417)	(67,855)
Cash dividends on preferred stock		(10,784)	(9,347)	(8,903)
Net cash used in financing activities		(178,540)	(138,687)	(439,466)
Net increase (decrease) in cash, cash equivalents, and restricted cash		(92,258)	(255,909)	(208,076)
Cash, cash equivalents, and restricted cash at beginning of period		146,877	402,786	610,862
Cash, cash equivalents, and restricted cash at end of period		$ 54,619	$ 146,877	$ 402,786