Investments (Unrealized Losses) (Details) $ in Thousands	Dec. 31, 2017 USD ($) investments	Dec. 31, 2016 USD ($)
Investment securities available for sale:
Less than 12 months, fair value	$ 3,752,642	$ 5,257,891
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	21,458	83,342
Aggregate fair value, continuous unrealized loss position, 12 months or more	2,975,631	362,351
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	59,470	4,689
Total fair value	6,728,273	5,620,242
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	80,928	88,031
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value	1,408,166	807,822
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	345	935
Aggregate fair value, continuous unrealized loss position, 12 months or more	249,698	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	201	0
Total fair value	1,657,864	807,822
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 546	935
Investment securities held to maturity:
Number of investments in continuous unrealized loss position for more than twelve months \| investments	4
Government Agency
Investment securities available for sale:
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Mortgage Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	$ 2,334,102	4,442,700
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	20,923	82,161
Aggregate fair value, continuous unrealized loss position, 12 months or more	2,725,933	362,351
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	59,269	4,689
Total fair value	5,060,035	4,805,051
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 80,192	86,850
Investment securities held to maturity:
Number of investments in continuous unrealized loss position for more than twelve months \| investments	227
Equity Securities
Investment securities available for sale:
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 0	0
Corporate Bonds
Investment securities available for sale:
Less than 12 months, fair value	5,025
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	8
Aggregate fair value, continuous unrealized loss position, 12 months or more	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Total fair value	5,025
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	0
Other Debt Obligations
Investment securities available for sale:
Less than 12 months, fair value	5,349	7,369
Available for Sale Securities Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses Accumulated In Investments	182	246
Aggregate fair value, continuous unrealized loss position, 12 months or more	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Total fair value	5,349	7,369
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 182	$ 246

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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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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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