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Investments (Unrealized Losses) (Details) - USD ($) | Mar. 31, 2018 | Dec. 31, 2017
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Schedule of Investments [Line Items] |  |
Unrealized Losses Related to Marketability of Securities or Issuer's Ability to Honor Redemption Obligations | $ 0 | $ 0
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 3,880,315,000 | 3,752,642,000
Unrealized Losses, Less than 12 months | 62,816,000 | 21,458,000
Fair Value, 12 months or more | 2,637,746,000 | 2,975,631,000
Unrealized Losses, Greater than 12 months | 96,056,000 | 59,470,000
Fair Value, Total | 6,518,061,000 | 6,728,273,000
Unrealized Losses, Total | 158,872,000 | 80,928,000
U. S. Treasury |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 1,453,968,000 | 1,408,166,000
Unrealized Losses, Less than 12 months | 4,049,000 | 345,000
Fair Value, 12 months or more | 99,925,000 | 249,698,000
Unrealized Losses, Greater than 12 months | 49,000 | 201,000
Fair Value, Total | 1,553,893,000 | 1,657,864,000
Unrealized Losses, Total | 4,098,000 | 546,000
Government Agency |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 12,059,000 | 848,000
Unrealized Losses, Less than 12 months | 81,000 | 12,000
Fair Value, 12 months or more | 2,159,000 | 2,527,000
Unrealized Losses, Greater than 12 months | 28,000 | 28,000
Fair Value, Total | 14,218,000 | 3,375,000
Unrealized Losses, Total | 109,000 | 40,000
Mortgage Backed Securities |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 2,404,338,000 | 2,333,254,000
Unrealized Losses, Less than 12 months | 58,606,000 | 20,911,000
Fair Value, 12 months or more | 2,535,662,000 | 2,723,406,000
Unrealized Losses, Greater than 12 months | 95,979,000 | 59,241,000
Fair Value, Total | 4,940,000,000 | 5,056,660,000
Unrealized Losses, Total | 154,585,000 | 80,152,000
Corporate bonds |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months |  | 5,025,000
Unrealized Losses, Less than 12 months |  | 8,000
Fair Value, 12 months or more |  | 0
Unrealized Losses, Greater than 12 months |  | 0
Fair Value, Total |  | 5,025,000
Unrealized Losses, Total |  | 8,000
Equity Securities |  |
Investment securities available for sale: |  |
Unrealized Losses, Total |  | 0
Corporate Bonds |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 9,950,000 |
Unrealized Losses, Less than 12 months | 80,000 |
Fair Value, 12 months or more | 0 |
Unrealized Losses, Greater than 12 months | 0 |
Fair Value, Total | 9,950,000 |
Unrealized Losses, Total | 80,000 | 8,000
Other |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months |  | 5,349,000
Unrealized Losses, Less than 12 months |  | 182,000
Fair Value, 12 months or more |  | 0
Unrealized Losses, Greater than 12 months |  | 0
Fair Value, Total |  | 5,349,000
Unrealized Losses, Total | $ 0 | $ 182,000

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