Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 6,245.9	$ 5,657.1
Fair Value	6,161.5	5,749.3
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	879.0	910.1
Gross Unrealized Gains	16.6	29.7
Gross Unrealized Loss	9.8	5.8
Fair Value	885.8	934.0
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	6,245.9	5,657.1
Gross Unrealized Gains	49.8	130.6
Gross Unrealized Loss	134.2	38.4
Fair Value	6,161.5	5,749.3
OTTI Unrealized Losses	6.6	6.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	6,270.8	7,688.8
Gross Unrealized Gains	49.8	145.2
Gross Unrealized Loss	134.6	54.3
Fair Value	6,186.0	7,779.7
OTTI Unrealized Losses	6.6	6.9
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	24.9	2,031.7
Gross Unrealized Gains		14.6
Gross Unrealized Loss	0.4	15.9
Fair Value	24.5	2,030.4
OTTI Unrealized Losses		0.4
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	414.7	322.1
Gross Unrealized Gains	2.4	3.4
Gross Unrealized Loss	7.2	3.2
Fair Value	409.9	322.3
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	7.3	8.2
Gross Unrealized Gains	0.1	0.3
Fair Value	7.4	8.5
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	3,476.6	3,134.2
Gross Unrealized Gains	26.1	84.1
Gross Unrealized Loss	92.0	21.7
Fair Value	3,410.7	3,196.6
OTTI Unrealized Losses	6.6	6.5
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	728.4	632.3
Gross Unrealized Gains	2.7	5.8
Gross Unrealized Loss	14.7	4.9
Fair Value	716.4	633.2
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	648.4	590.3
Gross Unrealized Gains	1.7	7.2
Gross Unrealized Loss	9.8	2.5
Fair Value	640.3	595.0
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	91.5	59.9
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Loss	0.7	0.3
Fair Value	$ 91.0	$ 59.7

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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