Investments (Amortized Cost, Gross Unrealized Gains And Losses, Fair Value And OTTI) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	$ 8,756.5	$ 8,870.8
Fixed maturity securities, fair value	9,662.6	9,572.1
Equity securities available for sale, cost or amortized cost	316.3	381.8
Equity securities available for sale, at fair value	368.0	421.4
Fixed maturity securities
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	8,756.5	8,870.8
Fixed maturity securities available for sale, gross unrealized gains	924.8	734.2
Fixed maturity securities available for sale, unrealized losses	(18.7)	(32.9)
Fixed maturity securities, fair value	9,662.6	9,572.1
OTTI in AOCI	27.6	31.7
Fixed maturity securities \| U.S. government and government agencies and authorities
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	180.6	172.8
Fixed maturity securities available for sale, gross unrealized gains	3.2	3.4
Fixed maturity securities available for sale, unrealized losses	(1.2)	(1.3)
Fixed maturity securities, fair value	182.6	174.9
OTTI in AOCI	0.0	0.0
Fixed maturity securities \| States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	302.3	446.9
Fixed maturity securities available for sale, gross unrealized gains	24.0	29.6
Fixed maturity securities available for sale, unrealized losses	(0.1)	(0.4)
Fixed maturity securities, fair value	326.2	476.1
OTTI in AOCI	0.0	0.0
Fixed maturity securities \| Foreign governments
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	524.8	508.9
Fixed maturity securities available for sale, gross unrealized gains	72.3	60.5
Fixed maturity securities available for sale, unrealized losses	(0.3)	(0.9)
Fixed maturity securities, fair value	596.8	568.5
OTTI in AOCI		0.0
Fixed maturity securities \| Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	188.4	2.6
Fixed maturity securities available for sale, gross unrealized gains	1.9	1.2
Fixed maturity securities available for sale, unrealized losses	(0.1)	(0.1)
Fixed maturity securities, fair value	190.2	3.7
OTTI in AOCI	1.0	1.1
Fixed maturity securities \| Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	38.6	39.3
Fixed maturity securities available for sale, gross unrealized gains	0.2	0.1
Fixed maturity securities available for sale, unrealized losses	(0.7)	(1.0)
Fixed maturity securities, fair value	38.1	38.4
OTTI in AOCI	0.0	0.0
Fixed maturity securities \| Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	1,084.2	1,071.2
Fixed maturity securities available for sale, gross unrealized gains	32.5	38.1
Fixed maturity securities available for sale, unrealized losses	(7.3)	(8.0)
Fixed maturity securities, fair value	1,109.4	1,101.3
OTTI in AOCI	9.2	12.8
Fixed maturity securities \| U.S. corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	4,774.2	5,022.7
Fixed maturity securities available for sale, gross unrealized gains	602.1	454.1
Fixed maturity securities available for sale, unrealized losses	(5.0)	(15.6)
Fixed maturity securities, fair value	5,371.3	5,461.2
OTTI in AOCI	17.4	15.6
Fixed maturity securities \| Foreign corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturity securities available for sale, amortized cost	1,663.4	1,606.4
Fixed maturity securities available for sale, gross unrealized gains	188.6	147.2
Fixed maturity securities available for sale, unrealized losses	(4.0)	(5.6)
Fixed maturity securities, fair value	1,848.0	1,748.0
OTTI in AOCI	0.0	2.2
Equity securities
Schedule of Available-for-sale Securities [Line Items]
OTTI in AOCI	0.0	0.0
Equity securities available for sale, cost or amortized cost	316.3	381.8
Equity securities available for sale, gross unrealized gains	52.2	40.7
Equity securities available for sale, gross unrealized losses	(0.5)	(1.1)
Equity securities available for sale, at fair value	368.0	421.4
Equity securities \| Common stocks
Schedule of Available-for-sale Securities [Line Items]
OTTI in AOCI	0.0	0.0
Equity securities available for sale, cost or amortized cost	9.3	11.9
Equity securities available for sale, gross unrealized gains	8.4	8.9
Equity securities available for sale, gross unrealized losses	0.0	0.0
Equity securities available for sale, at fair value	17.7	20.8
Equity securities \| Non-redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
OTTI in AOCI	0.0	0.0
Equity securities available for sale, cost or amortized cost	307.0	369.9
Equity securities available for sale, gross unrealized gains	43.8	31.8
Equity securities available for sale, gross unrealized losses	(0.5)	(1.1)
Equity securities available for sale, at fair value	$ 350.3	$ 400.6

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