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Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 5,097.6 | $ 5,039.7 | $ 3,360.2 | $ 4,050.9
Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 115.2 |  |  |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 249.0 | 283.9 |  |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 270.1 | 278.3 |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 86.1 | 89.8 |  |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 605.2 | 652.0 |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 309.9 | 205.3 |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 126.9 | 127.5 |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 436.8 | 332.8 |  |
Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1,042.0 | 984.8 |  |
Collateralized Credit Card Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 383.0 | 438.3 |  |
Collateralized Auto Loans |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 353.0 | 479.5 |  |
Collateralized Securities, Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 238.4 | 230.3 |  |
Asset-backed Securities, Securitized Loans and Receivables |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 974.4 | 1,148.1 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,016.4 | 2,132.9 |  |
Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,155.8 | 4,118.5 | $ 2,566.1 | $ 2,531.4
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 249.0 | 283.9 |  |
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 270.1 | 278.3 |  |
Level 2 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 86.1 | 89.8 |  |
Level 2 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 605.2 | 652.0 |  |
Level 2 Inputs | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 251.6 | 205.3 |  |
Level 2 Inputs | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 126.9 | 127.5 |  |
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 378.5 | 332.8 |  |
Level 2 Inputs | Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 983.7 | 984.8 |  |
Level 2 Inputs | Collateralized Credit Card Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 383.0 | 438.3 |  |
Level 2 Inputs | Collateralized Auto Loans |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 353.0 | 479.5 |  |
Level 2 Inputs | Collateralized Securities, Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 238.4 | 230.3 |  |
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 974.4 | 1,148.1 |  |
Level 2 Inputs | Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1,958.1 | 2,132.9 |  |
Level 3 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 58.3 | 0.0 |  |
Level 3 Inputs | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 58.3 | 0.0 |  |
Level 3 Inputs | Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 58.3 | 0.0 |  |
Level 3 Inputs | Collateralized Credit Card Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Collateralized Auto Loans |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Collateralized Securities, Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Asset-backed Securities, Securitized Loans and Receivables |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 58.3 | $ 0.0 |  |

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