Parent Company Only Financial Information, Statement of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 188,301	$ 190,509	$ 188,692
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sales of investment securities, net	(8,133)	(494)	(564)
Gain on sales of assets and impairment write-downs, net	(1,592)	(10,288)	(5,413)
Other, net	(10,513)	(4,203)	9,274
Net cash provided by operating activities	301,744	199,509	477,958
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of investment securities	1,601,947	102,011	136,152
Available for sale securities	(2,859,793)	(1,096,410)	(1,954,936)
Net cash used in investing activities	(1,534,356)	(2,118,605)	(1,253,234)
CASH FLOWS FROM FINANCING ACTIVITIES
Net increase (decrease) in short-term funding	(233,872)	327,362	(1,586,013)
Proceeds from issuance of long-term funding	250,000	996,030	2,500,000
Repayment of long-term funding	(1,500,035)	(155,066)	(427,430)
Proceeds from issuance of preferred shares	62,966	0	0
Proceeds from issuance of common stock for stock-based compensation plans	20,054	13,228	9,305
Purchase of preferred stock	(1,335)	(2,451)	(1,626)
Purchase of common stock returned to authorized but unissued	(93,000)	(150,498)	0
Purchase of treasury stock	(5,154)	(112,312)	(123,349)
Net cash provided by financing activities	674,230	2,348,918	639,648
Net increase (decrease) in cash and cash equivalents	(558,382)	429,822	(135,628)
Cash and cash equivalents at beginning of period	1,032,067	602,245	737,873
Cash and cash equivalents at end of period	473,685	1,032,067	602,245
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	188,301	190,509	188,692
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income of subsidiaries	(17,455)	23,646	54,928
Gain on sales of investment securities, net	0	(214)	(456)
Gain on sales of assets and impairment write-downs, net	(5,673)	(5,609)	(1,007)
Other, net	(10,997)	(14,022)	(103,954)
Net cash provided by operating activities	154,176	194,310	138,203
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of investment securities	13,962	88,844	55,445
Available for sale securities	0	0	(23,101)
Net decrease in notes receivable	0	46,594	3,049
Purchase of other assets, net of disposals	11,964	10,930	2,407
Net cash used in investing activities	25,926	146,368	37,800
CASH FLOWS FROM FINANCING ACTIVITIES
Net increase (decrease) in short-term funding	(6,319)	8,813	14,239
Proceeds from issuance of long-term funding	0	496,030	0
Repayment of long-term funding	0	(155,000)	(25,821)
Proceeds from issuance of preferred shares	62,966	0	0
Purchase of preferred stock	(1,335)	(2,451)	(1,626)
Cash dividends	(69,555)	(63,712)	(60,149)
Purchase of common stock returned to authorized but unissued	(93,000)	(150,498)	0
Purchase of treasury stock	(5,154)	(112,312)	(123,349)
Net cash provided by financing activities	(92,343)	34,098	(187,401)
Net increase (decrease) in cash and cash equivalents	87,759	374,776	(11,398)
Cash and cash equivalents at beginning of period	523,103	148,327	159,725
Cash and cash equivalents at end of period	$ 610,862	$ 523,103	$ 148,327