Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014 Senior Revolving Credit	Jun. 30, 2014 Commercial Paper Program	Jun. 30, 2014 Reported Value Measurement [Member]	Dec. 31, 2013 Reported Value Measurement [Member]	Jun. 30, 2014 Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Federal Home Loan Bank, advances, short-term	$ 250.0		$ 250.0				$ 250.0	$ 400.0
Debt instrument interest rate									0.20%
Senior unsecured notes, carrying value	12,685.0		12,685.0
Senior unsecured notes, principal amount repurchased	17.9		39.1
Senior unsecured notes, repurchase price	21.2		45.4
Loss on extinguishment of debt	3.0	0	6.0	0
Unsecured surplus note, carrying amount	24.9		24.9
Line of credit facility, maximum borrowing capacity					2,000.0
Line of credit maturity date					Sep. 29, 2016
Commercial paper authorized						2,500.0
Commercial paper						$ 867.1	$ 867.1	$ 379.2