Debt (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended						9 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Senior Revolving Credit	Sep. 30, 2014 Commercial Paper Program	Aug. 12, 2014 Senior Unsecured Notes [Member] Two Point Two Five Zero Percent Due Two Thousand Nineteen [Member]	Aug. 12, 2014 Senior Unsecured Notes [Member] Three Point Five Zero Zero Percent Due Two Thousand Twenty Four [Member]	Aug. 12, 2014 Senior Unsecured Notes [Member] Four Point Six Five Zero Percent Due Two Thousand Forty Four [Member]	Aug. 12, 2014 Senior Unsecured Notes [Member] Four Point Eight Five Zero Percent Due Two Thousand Fifty Four [Member]	Sep. 30, 2014 Senior Unsecured Notes [Member] Five Point Zero Zero Zero Percent Due Two Thousand Fourteen [Member]	Sep. 15, 2014 Senior Unsecured Notes [Member] Five Point Zero Zero Zero Percent Due Two Thousand Fourteen [Member]	Sep. 30, 2014 Senior Unsecured Notes [Member] Five Point Two Five Zero Percent Due Two Thousand Sixteen [Member]	Sep. 11, 2014 Senior Unsecured Notes [Member] Five Point Two Five Zero Percent Due Two Thousand Sixteen [Member]	Sep. 30, 2014 Senior Unsecured Notes [Member] Other senior unsecured notes [Member]	Sep. 30, 2014 Senior Unsecured Notes [Member] Other senior unsecured notes [Member]	Sep. 30, 2014 Reported Value Measurement [Member]	Dec. 31, 2013 Reported Value Measurement [Member]	Sep. 30, 2014 Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Debt instrument, repurchased face amount												$ 500.0		$ 1,097.9
Debt instrument interest rate							2.25%	3.50%	4.65%	4.85%		5.00%		5.25%					0.20%
Senior unsecured notes, repurchase price											512.3		1,178.2		13.6	59.1
Loss on extinguishment of debt	74.8	145.3	80.8	145.3							2.3		67.6		4.9	10.9
Senior unsecured notes, principal amount repurchased															11.3	50.4
Debt Instrument, face amount issued	1,500.0		1,500.0				850.0	800.0	800.0	250.0
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						425.8											425.8	379.2
Federal Home Loan Bank, advances, short-term	$ 100.0		$ 100.0														$ 100.0	$ 400.0