Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Investment securities available for sale
Cost	$ 7,156,448	$ 7,079,287
Gross Unrealized Gains	25,790	15,324
Gross Unrealized Losses	62,377	88,031
Fair Value	7,119,861	7,006,580
Investment securities held to maturity
Cost	83	98
U. S. Treasury
Investment securities available for sale
Cost	1,619,380	1,650,675
Gross Unrealized Gains	342	579
Gross Unrealized Losses	1,554	935
Fair Value	1,618,168	1,650,319
Government Agency
Investment securities available for sale
Cost	40,185	40,291
Gross Unrealized Gains	36	107
Gross Unrealized Losses	0	0
Fair Value	40,221	40,398
Mortgage Backed Securities
Investment securities available for sale
Cost	5,363,747	5,259,466
Gross Unrealized Gains	2,946	2,809
Gross Unrealized Losses	60,586	86,850
Fair Value	5,306,107	5,175,425
Investment securities held to maturity
Cost	83	98
Held To Maturity Securities Accumulated Unrecognized Holding Gain	6	6
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	89	104
Equity Securities
Investment securities available for sale
Cost	72,064	71,873
Gross Unrealized Gains	22,201	11,634
Gross Unrealized Losses	0	0
Fair Value	94,265	83,507
Corporate Bonds
Investment securities available for sale
Cost	49,370	49,367
Gross Unrealized Gains	220	195
Gross Unrealized Losses	25	0
Fair Value	49,565	49,562
Other Debt Obligations
Investment securities available for sale
Cost	11,702	7,615
Gross Unrealized Gains	45	0
Gross Unrealized Losses	212	246
Fair Value	$ 11,535	$ 7,369

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

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