Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment securities available for sale
Cost	$ 6,648,389	$ 6,885,797
Gross Unrealized Gains	42,666	6,367
Gross Unrealized Losses	3,766	30,871
Fair Value	6,687,289	6,861,293
Investment securities held to maturity
Cost	194	255
U. S. Treasury
Investment securities available for sale
Cost	1,538,907	1,675,996
Gross Unrealized Gains	2,308	4
Gross Unrealized Losses	0	1,118
Fair Value	1,541,215	1,674,882
Government Agency
Investment securities available for sale
Cost	355,488	498,804
Gross Unrealized Gains	669	230
Gross Unrealized Losses	0	374
Fair Value	356,157	498,660
Mortgage Backed Securities
Investment securities available for sale
Cost	4,693,313	4,692,447
Gross Unrealized Gains	37,375	5,120
Gross Unrealized Losses	3,406	29,369
Fair Value	4,727,282	4,668,198
Investment securities held to maturity
Cost	194	255
Held To Maturity Securities Accumulated Unrecognized Holding Gain	9	10
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	203	265
Equity Securities
Investment securities available for sale
Cost	50,066	7,935
Gross Unrealized Gains	2,314	968
Gross Unrealized Losses	285	10
Fair Value	52,095	8,893
Other Debt Obligations
Investment securities available for sale
Cost	10,615	10,615
Gross Unrealized Gains	0	45
Gross Unrealized Losses	75	0
Fair Value	$ 10,540	$ 10,660

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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