Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	$ 454.4	$ 1,078.9
United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	22.4
Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	368.6
Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	63.4
Maturity Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	448.3
Maturity Overnight and Continuous [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	16.3
Maturity Overnight and Continuous [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	368.6
Maturity Overnight and Continuous [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	63.4
Maturity Less than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Less than 30 Days [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Less than 30 Days [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Less than 30 Days [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity 30 to 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	6.1
Maturity 30 to 90 Days [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	6.1
Maturity 30 to 90 Days [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity 30 to 90 Days [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Greater than 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Greater than 90 Days [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Greater than 90 Days [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	0.0
Maturity Greater than 90 Days [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	$ 0.0