Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 7,100.4	$ 6,934.0
Gross Unrealized Gains	322.4	164.3
Gross Unrealized Loss	22.6	114.9
Fair Value	7,400.2	6,983.4
OTTI Unrealized Losses	16.1	27.8
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	311.1	447.1
Gross Unrealized Gains	9.4	5.5
Gross Unrealized Loss	0.1	3.5
Fair Value	320.4	449.1
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	234.5	244.7
Gross Unrealized Gains	8.2	2.6
Gross Unrealized Loss		1.5
Fair Value	242.7	245.8
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	1,071.8	1,074.5
Gross Unrealized Gains	65.7	50.0
Gross Unrealized Loss	1.1	4.2
Fair Value	1,136.4	1,120.3
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	3,937.4	3,699.9
Gross Unrealized Gains	190.5	86.8
Gross Unrealized Loss	19.7	95.7
Fair Value	4,108.2	3,691.0
OTTI Unrealized Losses	15.8	27.5
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	921.8	887.6
Gross Unrealized Gains	23.5	13.4
Gross Unrealized Loss	1.0	4.9
Fair Value	944.3	896.1
OTTI Unrealized Losses	0.3	0.3
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	546.5	499.6
Gross Unrealized Gains	23.7	5.8
Gross Unrealized Loss	0.4	4.3
Fair Value	569.8	501.1
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	77.3	80.6
Gross Unrealized Gains	1.4	0.2
Gross Unrealized Loss	0.3	0.8
Fair Value	78.4	80.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	533.6	528.5
Gross Unrealized Gains	79.8	55.7
Gross Unrealized Loss	0.9	7.6
Fair Value	$ 612.5	$ 576.6

Available For Sale Debt Securities OTTI Unrealized Loss

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Amount of accumulated pre-tax unrealized gains before deducting pre-tax unrealized losses on investments in available-for-sale securities impacting investments.

+ References

No definition available.

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Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale securities impacting investments.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

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+ 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 210.5-02.2)
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