Investment Securities (Summary Of Amount Of Gross Unrealized Losses For Debt Securities And Estimated Fair Value) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	$ 12,632	$ 42,879
Available-for-sale, Less than 12 months - Estimated fair value	2,686,960	4,904,774
Available-for-sale, 12 months or more - Gross unrealized losses	15,498	13,819
Available-for-sale, 12 months or more - Estimated fair value	832,896	750,132
Available-for-sale, Total - Gross unrealized losses	28,130	56,698
Available-for-sale, Total - Estimated fair value	3,519,856	5,654,906
Investment Securities Gross Unrealized Losses Less Than 12 Months	18,584	47,400
Investment Securities Estimated Fair Value Less Than 12 Months	2,880,697	5,026,971
Investment Securities Gross Unrealized Losses 12 Months or More	16,227	14,076
Investment Securities Estimated Fair Value 12 Months or More	845,376	763,944
Investment Securities Gross Unrealized Losses Total	34,811	61,476
Investment Securities Estimated Fair Value Total	3,726,073	5,790,915
Municipal Securities [Member]
Investment [Line Items]
Held-to-maturity, Less than 12 months - Gross unrealized losses	5,952	4,521
Held-to-maturity, Less than 12 months - Estimated fair value	193,737	122,197
Held-to-maturity, 12 months or more - Gross unrealized losses	729	257
Held-to-maturity, 12 months or more - Estimated fair value	12,480	13,812
Held-to-maturity, Total - Gross unrealized losses	6,681	4,778
Held-to-maturity, Total - Estimated fair value	206,217	136,009
Available-for-sale, Less than 12 months - Gross unrealized losses	935	735
Available-for-sale, Less than 12 months - Estimated fair value	140,612	102,901
Available-for-sale, 12 months or more - Gross unrealized losses	255	121
Available-for-sale, 12 months or more - Estimated fair value	14,231	5,733
Available-for-sale, Total - Gross unrealized losses	1,190	856
Available-for-sale, Total - Estimated fair value	154,843	108,634
Available-for-sale Securities [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	0
Available-for-sale, Less than 12 months - Estimated fair value	0
Agency Securities [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	312	2,176
Available-for-sale, Less than 12 months - Estimated fair value	299,591	559,196
Available-for-sale, 12 months or more - Gross unrealized losses	544	482
Available-for-sale, 12 months or more - Estimated fair value	129,191	131,615
Available-for-sale, Total - Gross unrealized losses	856	2,658
Available-for-sale, Total - Estimated fair value	428,782	690,811
Agency Guaranteed Mortgage-Backed Securities [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	6,836	34,583
Available-for-sale, Less than 12 months - Estimated fair value	1,707,106	3,639,824
Available-for-sale, 12 months or more - Gross unrealized losses	2,255	1,454
Available-for-sale, 12 months or more - Estimated fair value	149,293	65,071
Available-for-sale, Total - Gross unrealized losses	9,091	36,037
Available-for-sale, Total - Estimated fair value	1,856,399	3,704,895
Small Business Administration Loan-Backed Securities [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	4,549	5,348
Available-for-sale, Less than 12 months - Estimated fair value	539,651	567,365
Available-for-sale, 12 months or more - Gross unrealized losses	8,772	9,097
Available-for-sale, 12 months or more - Estimated fair value	528,850	535,376
Available-for-sale, Total - Gross unrealized losses	13,321	14,445
Available-for-sale, Total - Estimated fair value	1,068,501	1,102,741
Mutual Funds And Other [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	0	37
Available-for-sale, Less than 12 months - Estimated fair value	0	35,488
Available-for-sale, 12 months or more - Gross unrealized losses	0	0
Available-for-sale, 12 months or more - Estimated fair value	0	0
Available-for-sale, Total - Gross unrealized losses	0	37
Available-for-sale, Total - Estimated fair value	0	35,488
Available-for-sale Securities [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses	12,632	42,842
Available-for-sale, Less than 12 months - Estimated fair value	2,686,960	4,869,286
Available-for-sale, 12 months or more - Gross unrealized losses	15,498	13,819
Available-for-sale, 12 months or more - Estimated fair value	832,896	750,132
Available-for-sale, Total - Gross unrealized losses	28,130	56,661
Available-for-sale, Total - Estimated fair value	3,519,856	5,619,418
Available-for-sale Securities [Member] | Municipal Securities [Member]
Investment [Line Items]
Available-for-sale, 12 months or more - Gross unrealized losses	3,672
Available-for-sale, 12 months or more - Estimated fair value	11,331
Available-for-sale, Total - Gross unrealized losses	3,672	2,665
Available-for-sale, Total - Estimated fair value	$ 11,331	12,337
Asset-backed Securities [Member] | Other Credit Derivatives [Member]
Investment [Line Items]
Available-for-sale, Less than 12 months - Gross unrealized losses		0
Available-for-sale, Less than 12 months - Estimated fair value		0
Available-for-sale, 12 months or more - Gross unrealized losses		2,665
Available-for-sale, 12 months or more - Estimated fair value		$ 12,337