Investments (Current and Long-term Equity Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Equity Securities	$ 2,468.2	$ 3,632.0
Exchange Traded Funds [Member]
Investment [Line Items]
Equity Securities	631.0	1,300.3
Fixed Maturity Mutual Funds [Member]
Investment [Line Items]
Equity Securities	599.9	790.6
Common Equity Securities [Member]
Investment [Line Items]
Equity Securities	925.2	1,253.7
Private Equity Securities [Member]
Investment [Line Items]
Equity Securities	312.1	287.4
Equity Securities [Member]
Investment [Line Items]
Equity Securities	2,468.2	3,632.0
Equity Securities [Member] | Exchange Traded Funds [Member]
Investment [Line Items]
Equity Securities	631.0	1,300.3
Equity Securities [Member] | Fixed Maturity Mutual Funds [Member]
Investment [Line Items]
Equity Securities	599.9	790.6
Equity Securities [Member] | Common Equity Securities [Member]
Investment [Line Items]
Equity Securities	925.2	1,253.7
Equity Securities [Member] | Private Equity Securities [Member]
Investment [Line Items]
Equity Securities	$ 312.1	$ 287.4