Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment securities available for sale
Cost	$ 6,622,617	$ 7,079,287
Gross Unrealized Gains	29,404	15,324
Gross Unrealized Losses	55,571	88,031
Fair Value	6,596,450	7,006,580
Investment securities held to maturity
Cost	80	98
U. S. Treasury
Investment securities available for sale
Cost	1,619,225	1,650,675
Gross Unrealized Gains	0	579
Gross Unrealized Losses	3,091	935
Fair Value	1,616,134	1,650,319
Government Agency
Investment securities available for sale
Cost	40,080	40,291
Gross Unrealized Gains	1	107
Gross Unrealized Losses	0	0
Fair Value	40,081	40,398
Mortgage Backed Securities
Investment securities available for sale
Cost	4,822,517	5,259,466
Gross Unrealized Gains	1,951	2,809
Gross Unrealized Losses	52,226	86,850
Fair Value	4,772,242	5,175,425
Investment securities held to maturity
Cost	80	98
Held To Maturity Securities Accumulated Unrecognized Holding Gain	6	6
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	86	104
Equity Securities
Investment securities available for sale
Cost	78,753	71,873
Gross Unrealized Gains	27,106	11,634
Gross Unrealized Losses	0	0
Fair Value	105,859	83,507
Corporate Bonds
Investment securities available for sale
Cost	54,412	49,367
Gross Unrealized Gains	331	195
Gross Unrealized Losses	38	0
Fair Value	54,705	49,562
Other Debt Obligations
Investment securities available for sale
Cost	7,630	7,615
Gross Unrealized Gains	15	0
Gross Unrealized Losses	216	246
Fair Value	$ 7,429	$ 7,369

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

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