Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Investment income	$ 142.9	$ 119.3	$ 410.9	$ 352.7
Investment expenses	(5.8)	(4.8)	(18.0)	(14.9)
Net Investment Income	137.1	114.5	392.9	337.8
Fixed maturities
Net Investment Income [Line Items]
Investment income	106.5	87.7	308.7	262.5
Fixed maturities \| U.S. government obligations
Net Investment Income [Line Items]
Investment income	18.4	3.3	48.1	12.4
Fixed maturities \| State and local government obligations
Net Investment Income [Line Items]
Investment income	12.9	12.6	39.3	39.3
Fixed maturities \| Foreign government obligations
Net Investment Income [Line Items]
Investment income	0.1	0.1	0.3	0.3
Fixed maturities \| Corporate debt securities
Net Investment Income [Line Items]
Investment income	32.2	27.3	93.1	81.9
Fixed maturities \| Residential mortgage-backed securities
Net Investment Income [Line Items]
Investment income	7.8	11.1	27.3	34.9
Fixed maturities \| Agency residential pass-through obligations
Net Investment Income [Line Items]
Investment income	0.2	0.3	0.6	0.9
Fixed maturities \| Commercial mortgage-backed securities
Net Investment Income [Line Items]
Investment income	20.0	22.4	57.1	62.6
Fixed maturities \| Other asset-backed securities
Net Investment Income [Line Items]
Investment income	12.1	6.8	33.8	18.7
Fixed maturities \| Redeemable preferred stocks
Net Investment Income [Line Items]
Investment income	2.8	3.8	9.1	11.5
Equity securities \| Nonredeemable preferred stocks
Net Investment Income [Line Items]
Investment income	10.9	12.4	33.0	36.8
Equity securities \| Common equities
Net Investment Income [Line Items]
Investment income	14.9	13.9	42.8	42.0
Short-term investments
Net Investment Income [Line Items]
Investment income	$ 10.6	$ 5.3	$ 26.4	$ 11.4

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
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-Subparagraph (SX 210.5-03.9)
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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