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Investments (Unrealized Losses) (Details) - USD ($) | Jun. 30, 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 | 3,535,984,000 | 3,752,642,000
Unrealized Losses, Less than 12 months | 55,802,000 | 21,458,000
Fair Value, 12 months or more | 661,241,000 | 2,975,631,000
Unrealized Losses, Greater than 12 months | 21,136,000 | 59,470,000
Fair Value, Total | 4,197,225,000 | 6,728,273,000
Unrealized Losses, Total | 76,938,000 | 80,928,000
U. S. Treasury |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 1,503,974,000 | 1,408,166,000
Unrealized Losses, Less than 12 months | 4,461,000 | 345,000
Fair Value, 12 months or more | 0 | 249,698,000
Unrealized Losses, Greater than 12 months | 0 | 201,000
Fair Value, Total | 1,503,974,000 | 1,657,864,000
Unrealized Losses, Total | 4,461,000 | 546,000
Government Agency |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 83,383,000 | 848,000
Unrealized Losses, Less than 12 months | 328,000 | 12,000
Fair Value, 12 months or more | 2,004,000 | 2,527,000
Unrealized Losses, Greater than 12 months | 27,000 | 28,000
Fair Value, Total | 85,387,000 | 3,375,000
Unrealized Losses, Total | 355,000 | 40,000
Mortgage Backed Securities |  |
Investment securities available for sale: |  |
Fair Value, Less than 12 months | 1,923,954,000 | 2,333,254,000
Unrealized Losses, Less than 12 months | 50,765,000 | 20,911,000
Fair Value, 12 months or more | 654,212,000 | 2,723,406,000
Unrealized Losses, Greater than 12 months | 21,107,000 | 59,241,000
Fair Value, Total | 2,578,166,000 | 5,056,660,000
Unrealized Losses, Total | 71,872,000 | 80,152,000
Investment securities held to maturity |  |
Fair Value, Less than 12 months | 1,126,683,000 |
Unrealized Losses, Less than 12 months | 1,249,000 |
Fair Value, 12 months or more | 11,219,000 |
Unrealized Losses, 12 months or more | 210,000 |
Fair Value, Total | 1,137,902,000 |
Unrealized Losses, Total | 1,459,000 | 0
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 | 24,673,000 |
Unrealized Losses, Less than 12 months | 248,000 |
Fair Value, 12 months or more | 5,025,000 |
Unrealized Losses, Greater than 12 months | 2,000 |
Fair Value, Total | 29,698,000 |
Unrealized Losses, Total | 250,000 | 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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