Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 6,842.8	$ 6,934.0
Gross Unrealized Gains	330.2	164.3
Gross Unrealized Loss	31.0	114.9
Fair Value	7,142.0	6,983.4
OTTI Unrealized Losses	18.9	27.8
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	364.9	447.1
Gross Unrealized Gains	11.6	5.5
Gross Unrealized Loss		3.5
Fair Value	376.5	449.1
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	229.4	244.7
Gross Unrealized Gains	7.2	2.6
Gross Unrealized Loss		1.5
Fair Value	236.6	245.8
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	1,084.6	1,074.5
Gross Unrealized Gains	76.2	50.0
Gross Unrealized Loss	0.9	4.2
Fair Value	1,159.9	1,120.3
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	3,728.8	3,699.9
Gross Unrealized Gains	182.7	86.8
Gross Unrealized Loss	28.8	95.7
Fair Value	3,882.7	3,691.0
OTTI Unrealized Losses	18.6	27.5
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	849.0	887.6
Gross Unrealized Gains	26.1	13.4
Gross Unrealized Loss	0.9	4.9
Fair Value	874.2	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	507.5	499.6
Gross Unrealized Gains	25.5	5.8
Gross Unrealized Loss	0.1	4.3
Fair Value	532.9	501.1
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	78.6	80.6
Gross Unrealized Gains	0.9	0.2
Gross Unrealized Loss	0.3	0.8
Fair Value	79.2	80.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	507.8	528.5
Gross Unrealized Gains	76.8	55.7
Gross Unrealized Loss	1.9	7.6
Fair Value	$ 582.7	$ 576.6