Debt (Medium-Term Notes) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Proceeds from Issuance of Long-term Debt	$ 495.1	$ 3.3
6.11% Notes Due 2028 [Member]
Principal amount of note	8.5
Debt Instrument, Interest Rate, Stated Percentage	6.11%
Maturity date	Jul. 17, 2028
6.75% Senior Notes Due March 2012 [Member]
Principal amount of note	250.0
Debt Instrument, Interest Rate, Stated Percentage	6.75%
Debt Instrument, Effective Interest Rate During Period, Net Of Interest Rate Swaps Impact	3.50%
5.5% Notes Due April 2013 [Member]
Principal amount of note	500.0
Debt Instrument, Interest Rate, Stated Percentage	5.50%
2% Notes Due 2015 And 4% Notes Due 2022 [Member]
Principal amount of note	500.0
Debt Instrument, Principal Amount, Change In Control Redemption Percent	101.00%
Proceeds from Issuance of Long-term Debt	495.1
2% Notes Due June 2015 [Member]
Principal amount of note	250.0
Debt Instrument, Interest Rate, Stated Percentage	2.00%
Maturity date	Jun. 13, 2015
4% Notes Due June 2022 [Member]
Principal amount of note	$ 250.0
Debt Instrument, Interest Rate, Stated Percentage	4.00%
Maturity date	Jun. 13, 2022
Debt Instrument, Principal Amount, Redemption Percent	100.00%