Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment securities available for sale
Cost	$ 7,015,200	$ 7,229,014
Gross Unrealized Gains	1,412	32,094
Gross Unrealized Losses	158,872	80,928
Fair Value	6,857,740	7,180,180
Investment securities held to maturity
Cost	74	76
U. S. Treasury
Investment securities available for sale
Cost	1,557,991	1,658,410
Gross Unrealized Gains	0	0
Gross Unrealized Losses	4,098	546
Fair Value	1,553,893	1,657,864
Government Agency
Investment securities available for sale
Cost	32,006	8,695
Gross Unrealized Gains	12	15
Gross Unrealized Losses	109	40
Fair Value	31,909	8,670
Mortgage Backed Securities
Investment securities available for sale
Cost	5,360,251	5,419,379
Gross Unrealized Gains	1,001	1,529
Gross Unrealized Losses	154,585	80,152
Fair Value	5,206,667	5,340,756
Investment securities held to maturity
Cost	74	76
Held To Maturity Securities Accumulated Unrecognized Holding Gain	5	5
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	79	81
Equity Securities
Investment securities available for sale
Cost	0	75,471
Gross Unrealized Gains		29,737
Gross Unrealized Losses		0
Fair Value	0	105,208
Corporate Bonds
Investment securities available for sale
Cost	59,414	59,414
Gross Unrealized Gains	319	557
Gross Unrealized Losses	80	8
Fair Value	59,653	59,963
Other
Investment securities available for sale
Cost	5,538	7,645
Gross Unrealized Gains	80	256
Gross Unrealized Losses	0	182
Fair Value	$ 5,618	$ 7,719

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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