Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	$ 1,078.9	$ 1,300.9
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	161.6
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	658.8
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	256.8
Other Debt 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	1.7
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	995.5
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	79.5
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	658.8
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	255.5
Maturity Overnight and Continuous [Member] | Other Debt 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	1.7
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	26.5
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	25.2
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	1.3
Maturity Less than 30 Days [Member] | Other Debt 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	48.2
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	48.2
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 30 to 90 Days [Member] | Other Debt 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	8.7
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	8.7
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
Maturity Greater than 90 Days [Member] | Other Debt 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