Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	$ 906.4	$ 1,078.9
United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	34.8
Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	720.7
Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	150.9
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	897.7
Overnight and Continuous [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	26.1
Overnight and Continuous [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	720.7
Overnight and Continuous [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	150.9
Less than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	8.7
Less than 30 Days [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	8.7
Less than 30 Days [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	0.0
Less than 30 Days [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	$ 0.0