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Accounting Policies and Basis of Presentation Accounting Policies and Basis of Presentation (Recently Adopted Accounting Standards) (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 22, 2017 Rate | Mar. 31, 2018 USD ($) basis_points | Mar. 31, 2017 USD ($) | Dec. 31, 2017 Rate
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Federal Statutory Income Tax Rate, Percent | Rate | 21.00% |  |  | 35.00%
Merchant processing |  |  | $ 18.3 |
Cardholder processing |  |  | 6.8 |
Cardholder reward programs |  |  | $ 2.5 |
Income tax ASU cumulative effect adjustment |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | $ 31.3 |  |
Financial Instruments ASU cumulative effect adjustment |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  | 18.7 |  |
Minimum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | 65.0 |  |
Maximum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification |  | $ 85.0 |  |
Capital basis points | Minimum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Capital Quantification | basis_points |  | 4 |  |
Capital basis points | Maximum |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Capital Quantification | basis_points |  | 6 |  |

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