Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 7,847.8	$ 7,688.8
Fair Value	7,729.9	7,779.7
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	1,035.3	1,053.3
Gross Unrealized Gains	17.9	29.8
Gross Unrealized Loss	15.5	7.1
Fair Value	1,037.7	1,076.0
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	7,847.8	7,688.8
Gross Unrealized Gains	63.4	145.2
Gross Unrealized Loss	181.3	54.3
Fair Value	7,729.9	7,779.7
OTTI Unrealized Losses	6.6	6.9
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	595.1	513.6
Gross Unrealized Gains	1.7	3.4
Gross Unrealized Loss	16.3	5.6
Fair Value	580.5	511.4
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	209.6	240.8
Gross Unrealized Gains	2.1	3.2
Gross Unrealized Loss	2.2	1.3
Fair Value	209.5	242.7
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	4,278.7	4,238.9
Gross Unrealized Gains	38.9	95.0
Gross Unrealized Loss	101.3	26.4
Fair Value	4,216.3	4,307.5
OTTI Unrealized Losses	6.6	6.9
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	1,026.7	990.6
Gross Unrealized Gains	1.8	6.5
Gross Unrealized Loss	31.5	11.1
Fair Value	997.0	986.0
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	624.0	591.7
Gross Unrealized Gains	1.0	7.2
Gross Unrealized Loss	13.4	2.5
Fair Value	611.6	596.4
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	78.4	59.9
Gross Unrealized Gains		0.1
Gross Unrealized Loss	1.1	0.3
Fair Value	$ 77.3	$ 59.7