Parent Company Only Financials Statement of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 190,509	$ 188,692	$ 178,973
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	50,281	49,711	43,611
(Gain) loss on sales of investment securities, net of impairment write-downs	(494)	(564)	(4,261)
(Gain) loss on sales of assets and impairment write-downs, net	(10,288)	(5,413)	12,096
Net change in other assets and other liabilities	9,025	18,579	52,361
Net cash provided by (used in) operating activities	212,737	487,263	350,484
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of investment securities	102,011	136,152	299,782
Available for sale securities	(1,096,410)	(1,954,936)	(2,084,500)
Purchase of other assets, net of disposals	(4,496)	(2,098)	(5,968)
Net cash provided by (used in) investing activities	(2,118,605)	(1,253,234)	(1,633,722)
CASH FLOWS FROM FINANCING ACTIVITIES
Net increase (decrease) in short-term funding	327,362	(1,586,013)	(187,546)
Proceeds from issuance of long-term funding	996,030	2,500,000	154,738
Repayment of long-term funding	(155,066)	(427,430)	(311,621)
Purchase of preferred stock	(2,451)	(1,626)	0
Purchase of common stock returned to authorized but unissued	(150,498)	0	0
Purchase of treasury stock	(112,312)	(123,349)	(61,654)
Net cash provided by (used in) financing activities	2,335,690	630,343	1,404,516
Net increase (decrease) in cash and cash equivalents	429,822	(135,628)	121,278
Cash and cash equivalents at beginning of year	602,245	737,873	616,595
Cash and cash equivalents at end of year	1,032,067	602,245	737,873
Parent Company [Member]
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	190,509	188,692	178,973
Adjustments to reconcile net income to net cash provided by operating activities:
(Increase) decrease in equity in undistributed net income (loss) of subsidiaries	23,646	54,928	(33,931)
(Gain) loss on sales of investment securities, net of impairment write-downs	(214)	(456)	(530)
(Gain) loss on sales of assets and impairment write-downs, net	(5,609)	(1,007)	1,365
Net change in other assets and other liabilities	(794)	(94,649)	143,032
Net cash provided by (used in) operating activities	207,538	147,508	288,909
CASH FLOWS FROM INVESTING ACTIVITIES
Proceeds from sales of investment securities	88,844	55,445	49,819
Available for sale securities	0	(23,101)	(186,432)
Net (increase) decrease in notes receivable	46,594	3,049	(225)
Purchase of other assets, net of disposals	10,930	2,407	(7,336)
Net cash provided by (used in) investing activities	146,368	37,800	(144,174)
CASH FLOWS FROM FINANCING ACTIVITIES
Net increase (decrease) in short-term funding	8,813	14,239	51,245
Proceeds from issuance of long-term funding	496,030	0	154,738
Repayment of long-term funding	(155,000)	(25,821)	(211,340)
Purchase of preferred stock	(2,451)	(1,626)	0
Cash dividends	(63,712)	(60,149)	(44,834)
Purchase of common stock returned to authorized but unissued	(150,498)		0
Purchase of treasury stock	(112,312)	(123,349)	(61,654)
Net cash provided by (used in) financing activities	20,870	(196,706)	(111,845)
Net increase (decrease) in cash and cash equivalents	374,776	(11,398)	32,890
Cash and cash equivalents at beginning of year	148,327	159,725	126,835
Cash and cash equivalents at end of year	$ 523,103	$ 148,327	$ 159,725