Investments (Aggregate Values and Unrealized Gains and Losses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Aggregate fair value, continuous unrealized loss position, 12 months or more	$ 2,975,631	$ 362,351
Investment securities available for sale
Cost	7,229,014	7,079,287
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	32,094	15,324
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	80,928	88,031
Fair Value	7,180,180	7,006,580
Investment securities held to maturity
Cost	76	98
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	59,470	4,689
U. S. Treasury
Schedule of Investments [Line Items]
Aggregate fair value, continuous unrealized loss position, 12 months or more	249,698	0
Investment securities available for sale
Cost	1,658,410	1,650,675
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	579
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	546	935
Fair Value	1,657,864	1,650,319
Investment securities held to maturity
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	201	0
Government Agency
Investment securities available for sale
Cost		40,291
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax		107
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Fair Value		40,398
Mortgage Backed Securities
Schedule of Investments [Line Items]
Aggregate fair value, continuous unrealized loss position, 12 months or more	2,725,933	362,351
Investment securities available for sale
Cost	5,428,074	5,259,466
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,544	2,809
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	80,192	86,850
Fair Value	5,349,426	5,175,425
Investment securities held to maturity
Cost	76	98
Held To Maturity Securities Accumulated Unrecognized Holding Gain	5	6
Held To Maturity Securities Accumulated Unrecognized Holding Loss	0	0
Fair Value	81	104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	59,269	4,689
Equity Securities
Investment securities available for sale
Cost	75,471	71,873
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	29,737	11,634
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Fair Value	105,208	83,507
Corporate Bonds
Schedule of Investments [Line Items]
Aggregate fair value, continuous unrealized loss position, 12 months or more	0
Investment securities available for sale
Cost	59,414	49,367
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	557	195
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	0
Fair Value	59,963	49,562
Investment securities held to maturity
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Other Debt Obligations
Schedule of Investments [Line Items]
Aggregate fair value, continuous unrealized loss position, 12 months or more	0	0
Investment securities available for sale
Cost	7,645	7,615
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	256	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	182	246
Fair Value	7,719	7,369
Investment securities held to maturity
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	$ 0

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

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