Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,492.7	$ 6,245.9
Fair Value	6,559.4	6,161.5
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	848.1	879.0
Gross Unrealized Gains	20.8	16.6
Gross Unrealized Loss	3.4	9.8
Fair Value	865.5	885.8
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,245.9
Gross Unrealized Gains		49.8
Gross Unrealized Loss		134.2
Fair Value		6,161.5
OTTI Unrealized Losses		6.6
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		24.9
Gross Unrealized Loss		0.4
Fair Value		24.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,492.7	6,270.8
Gross Unrealized Gains	108.7	49.8
Gross Unrealized Loss	42.0	134.6
Fair Value	6,559.4	6,186.0
OTTI Unrealized Losses	4.8	6.6
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	404.5	414.7
Gross Unrealized Gains	4.7	2.4
Gross Unrealized Loss	2.5	7.2
Fair Value	406.7	409.9
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7.3	7.3
Gross Unrealized Gains	0.1	0.1
Fair Value	7.4	7.4
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,635.7	3,476.6
Gross Unrealized Gains	66.3	26.1
Gross Unrealized Loss	27.4	92.0
Fair Value	3,674.6	3,410.7
OTTI Unrealized Losses	4.8	6.6
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	819.2	728.4
Gross Unrealized Gains	6.8	2.7
Gross Unrealized Loss	6.1	14.7
Fair Value	819.9	716.4
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	684.9	648.4
Gross Unrealized Gains	9.4	1.7
Gross Unrealized Loss	2.4	9.8
Fair Value	691.9	640.3
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	93.0	91.5
Gross Unrealized Gains	0.6	0.2
Gross Unrealized Loss	0.2	0.7
Fair Value	$ 93.4	$ 91.0

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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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