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Investment Securities (Mortgage-backed Asset Securities Table) (Details) - USD ($) $ in Millions | Sep. 30, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 4,900.2 | $ 4,050.9 | $ 3,365.4 | $ 3,343.6
Commercial Mortgage Backed Securities Fixed Rate [Member] |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 102.3 |  |  |
Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 301.1 | 265.5 |  |
Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 239.4 | 42.2 |  |
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 35.5 | 22.8 |  |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 576.0 | 330.5 |  |
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 213.4 | 133.2 |  |
Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 113.7 | 140.4 |  |
Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 327.1 | 273.6 |  |
Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 903.1 | 604.1 |  |
Collateralized Credit Card Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 534.0 | 217.7 |  |
Collateralized Auto Loans |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 721.8 | 269.7 |  |
Collateralized Securities, Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 255.6 | 75.5 |  |
Asset-backed Securities, Securitized Loans and Receivables |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1,511.4 | 562.9 |  |
Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,414.5 | 1,167.0 |  |
Level 2 Inputs |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 4,044.8 | 2,531.4 | $ 2,561.6 | $ 2,372.9
Level 2 Inputs | Government National Mortgage Association Certificates and Obligations (GNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 301.1 | 265.5 |  |
Level 2 Inputs | Federal National Mortgage Association Certificates and Obligations (FNMA) |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 239.4 | 42.2 |  |
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 | 35.5 | 22.8 |  |
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 | 576.0 | 330.5 |  |
Level 2 Inputs | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 177.4 | 133.2 |  |
Level 2 Inputs | Commercial Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 113.7 | 140.4 |  |
Level 2 Inputs | Mortgage-backed Securities, Issued by Private Enterprises |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 291.1 | 273.6 |  |
Level 2 Inputs | Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 867.1 | 604.1 |  |
Level 2 Inputs | Collateralized Credit Card Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 534.0 | 217.7 |  |
Level 2 Inputs | Collateralized Auto Loans |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 721.8 | 269.7 |  |
Level 2 Inputs | Collateralized Securities, Other |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 255.6 | 75.5 |  |
Level 2 Inputs | Asset-backed Securities, Securitized Loans and Receivables |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 1,511.4 | 562.9 |  |
Level 2 Inputs | Mortgage-backed and asset-backed securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 2,378.5 | 1,167.0 |  |
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 | 36.0 | 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 | 36.0 | 0.0 |  |
Level 3 Inputs | Collateralized Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 36.0 | 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 | $ 36.0 | $ 0.0 |  |

X
- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 + Details
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