Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Fair value of collateral received at time of securities lending transactions	$ 1,300.9	$ 1,515.3
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	145.1
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	813.3
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	306.5
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	36.0
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	1,251.8
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	102.3
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	813.3
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	300.2
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	36.0
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	16.0
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	9.7
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	6.3
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	1.0
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	1.0
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	32.1
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	32.1
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