Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment securities available for sale
Cost	$ 7,004,629	$ 7,079,287
Gross Unrealized Gains	35,288	15,324
Gross Unrealized Losses	47,040	88,031
Fair Value	6,992,877	7,006,580
Investment securities held to maturity
Cost	78	98
U. S. Treasury
Investment securities available for sale
Cost	1,619,343	1,650,675
Gross Unrealized Gains	0	579
Gross Unrealized Losses	3,019	935
Fair Value	1,616,324	1,650,319
Government Agency
Investment securities available for sale
Cost		40,291
Gross Unrealized Gains		107
Gross Unrealized Losses		0
Fair Value		40,398
Mortgage Backed Securities
Investment securities available for sale
Cost	5,240,922	5,259,466
Gross Unrealized Gains	3,256	2,809
Gross Unrealized Losses	43,837	86,850
Fair Value	5,200,341	5,175,425
Investment securities held to maturity
Cost	78	98
Held To Maturity Securities Accumulated Unrecognized Holding Gain	6	6
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	84	104
Equity Securities
Investment securities available for sale
Cost	82,314	71,873
Gross Unrealized Gains	31,336	11,634
Gross Unrealized Losses	0	0
Fair Value	113,650	83,507
Corporate Bonds
Investment securities available for sale
Cost	54,412	49,367
Gross Unrealized Gains	471	195
Gross Unrealized Losses	10	0
Fair Value	54,873	49,562
Other Debt Obligations
Investment securities available for sale
Cost	7,638	7,615
Gross Unrealized Gains	225	0
Gross Unrealized Losses	174	246
Fair Value	$ 7,689	$ 7,369

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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