Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	$ 2,117.2
Trading Securities by Maturity, Fair Value
Trading Securities	2,129.7
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	109.0
Due after one year through five years	663.0
Due after five years through ten years	464.9
Due after ten years	183.1
Trading Securities by Maturity, Fair Value
Due in one year or less	108.8
Due after one year through five years	660.9
Due after five years through ten years	472.0
Due after ten years	193.3
Mortgage and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Mortgage and asset-backed securities	697.2
Trading Securities by Maturity, Fair Value
Mortgage and asset-backed securities	$ 694.7

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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