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Investments (Narrative) (Details) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) investments | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) investments | Jun. 30, 2017 USD ($) | May 01, 2018 USD ($) | Dec. 31, 2017 USD ($)
Schedule of Investments [Line Items] |  |  |  |  |  |
Marketable equity securities | $ 107,264,000 |  | $ 107,264,000 |  |  | $ 0
Unrealized Losses Related to Marketability of Securities or Issuer's Ability to Honor Redemption Obligations | 0 |  | 0 |  |  | 0
Fair Value, 12 months or more in unrealized loss position | 661,241,000 |  | 661,241,000 |  |  | 2,975,631,000
Unrealized Losses, Greater than 12 months | 21,136,000 |  | 21,136,000 |  |  | 59,470,000
Investment value deemed to be OTTI | 0 | $ 0 |  |  |  |
Investment securities, aggregate carrying value, pledged as collateral | 3,670,000,000 |  | 3,670,000,000 |  |  | 4,590,000,000
Investment securities available for sale | 4,783,507,000 |  | 4,783,507,000 |  |  | 7,180,180,000
Unrealized losses on available for sale securities transferred to held to maturity included in comprehensive income | (109,507,000) | $ 0 | (109,507,000) | $ 0 |  |
Equity Securities |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Investment securities available for sale | 0 |  | 0 |  |  | 105,208,000
Mortgage Backed Securities |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Fair Value, 12 months or more in unrealized loss position | 654,212,000 |  | 654,212,000 |  |  | 2,723,406,000
Unrealized Losses, Greater than 12 months | $ 21,107,000 |  | $ 21,107,000 |  |  | 59,241,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments | 119 |  | 119 |  |  |
Investment securities available for sale | $ 3,034,065,000 |  | $ 3,034,065,000 |  |  | 5,340,756,000
Investment securities available for sale transferred to held to maturity amortized cost |  |  |  |  | $ 2,490,000,000 |
Investment securities available for sale transferred to held to maturity fair value |  |  |  |  | $ 2,380,000,000 |
U. S. Treasury |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Fair Value, 12 months or more in unrealized loss position | 0 |  | 0 |  |  | 249,698,000
Unrealized Losses, Greater than 12 months | 0 |  | 0 |  |  | 201,000
Investment securities available for sale | 1,503,974,000 |  | 1,503,974,000 |  |  | 1,657,864,000
Corporate Bonds |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Fair Value, 12 months or more in unrealized loss position | 5,025,000 |  | 5,025,000 |  |  |
Unrealized Losses, Greater than 12 months | $ 2,000 |  | $ 2,000 |  |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | investments | 1 |  | 1 |  |  |
Investment securities available for sale | $ 108,790,000 |  | $ 108,790,000 |  |  | $ 59,963,000
Financial Instruments ASU cumulative effect adjustment |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | $ 18,700,000 |  | $ 18,700,000 |  |  |

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