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Investments (Narrative) (Details) shares in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) investment investments shares | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) investment investments shares | Sep. 30, 2017 USD ($) | May 01, 2018 USD ($) | Dec. 31, 2017 USD ($)
Schedule of Investments [Line Items] |  |  |  |  |  |
Investment in marketable equity securities | $ 109,907,000 |  | $ 109,907,000 |  |  | $ 0
Investment securities available for sale | 4,677,351,000 |  | 4,677,351,000 |  |  | 7,180,180,000
Unrealized losses on available for sale securities transferred to held to maturity included in comprehensive income | $ 0 | $ 0 | $ (109,507,000) | $ 0 |  |
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, number of positions | investment | 185 |  | 185 |  |  |
Held-to-maturity, securities in unrealized loss positions, qualitative disclosure, number of positions | investment | 2 |  | 2 |  |  |
Unrealized losses related to marketability of securities or issuer's ability to honor redemption obligations | $ 0 |  | $ 0 |  |  |
Investment value deemed to be OTTI | 0 |  |  |  |  |
Investment securities, aggregate carrying value, pledged as collateral | 3,530,000,000 |  | 3,530,000,000 |  |  | 4,590,000,000
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] |  |  |  |  |  |
Investment securities available for sale | $ 2,919,796,000 |  | $ 2,919,796,000 |  |  | 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 |
Available-for-sale, securities in unrealized loss positions, number of positions, greater than or equal to one year | investments | 183 |  | 183 |  |  |
U. S. Treasury |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Investment securities available for sale | $ 1,505,434,000 |  | $ 1,505,434,000 |  |  | 1,657,864,000
Corporate Bonds |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Investment securities available for sale | $ 119,933,000 |  | $ 119,933,000 |  |  | $ 59,963,000
Available-for-sale, securities in unrealized loss positions, number of positions, greater than or equal to one year | investments | 1 |  | 1 |  |  |
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 |  |  |
Visa Class B Common Stock |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Investment owned (in shares) | shares | 298 |  | 298 |  |  |
Investment owned, fair value | $ 0 |  | $ 0 |  |  |
U. S. Treasury |  |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |  |
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, number of positions | investment | 1 |  | 1 |  |  |

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