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Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 2,509.4 | $ 2,547.3 | $ 2,406.9 | $ 1,543.5
Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.1 | $ 70.3 |  |
Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 58.4 |  |  |
Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.7 |  |  |
Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 70.1 |  |  |
Prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 58.4 |  |  |
Prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 11.7 |  |  |
Prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Non-prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Non-prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Non-prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Non-prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Sub-prime | Residential Mortgage Backed Securities |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Sub-prime | Residential Mortgage Backed Securities | Super Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Sub-prime | Residential Mortgage Backed Securities | Senior |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Sub-prime | Residential Mortgage Backed Securities | Subordinate |  |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |  |
Fair value investments | $ 0.0 |  |  |

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- 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 -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(12)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 5: 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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 7: 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=65884222&loc=d3e22054-111558 Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 9: 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 + Details
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