Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,045.3	$ 1,053.3
Gross Unrealized Gains	21.8	29.8
Gross Unrealized Loss	12.0	7.1
Fair Value	1,055.1	1,076.0
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	546.7	513.6
Gross Unrealized Gains	2.0	3.4
Gross Unrealized Loss	12.7	5.6
Fair Value	536.0	511.4
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	244.7	240.8
Gross Unrealized Gains	2.3	3.2
Gross Unrealized Loss	2.2	1.3
Fair Value	244.8	242.7
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,279.0	4,238.9
Gross Unrealized Gains	49.8	95.0
Gross Unrealized Loss	78.1	26.4
Fair Value	4,250.7	4,307.5
OTTI Unrealized Losses	6.6	6.9
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,025.6	990.6
Gross Unrealized Gains	3.6	6.5
Gross Unrealized Loss	26.8	11.1
Fair Value	1,002.4	986.0
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	611.0	591.7
Gross Unrealized Gains	1.7	7.2
Gross Unrealized Loss	9.4	2.5
Fair Value	603.3	596.4
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	67.0	59.9
Gross Unrealized Gains		0.1
Gross Unrealized Loss	1.0	0.3
Fair Value	66.0	59.7
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,819.3	7,688.8
Gross Unrealized Gains	81.2	145.2
Gross Unrealized Loss	142.2	54.3
Fair Value	7,758.3	7,779.7
OTTI Unrealized Losses	$ 6.6	$ 6.9