Capital Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Sep. 30, 2011	Aug. 15, 2011	Jun. 13, 2011	May 17, 2011	Dec. 31, 2010	Jun. 30, 2010	Jun. 13, 2011 Common Stock [Member]	Jun. 13, 2011 Over-allotment Option [Member]	May 25, 2011 Preferred Class A [Member]	May 25, 2011 Preferred Class B [Member]	May 17, 2011 Maximum [Member]
Shares authorized to be repurchased					4,000,000
Stock repurchased during period, shares	3,000,000
Stock repurchased during period, value	$ 222
Treasury stock acquired, cost per share	$ 74
Remaining number of shares authorized to be repurchased	1,000,000			1,000,000
Common shares, issued	149,195,469			149,195,469		10,350,000		138,845,469		9,000,000	1,350,000
Price per share	$ 85.63			$ 85.63
Proceeds from issuance of common stock	$ 853.7			$ 853.7
Percentage increase in dividends payable				100.00%
Dividend payable, per share	$ 0.28			$ 0.28				$ 0.0875	$ 0.14
Common share dividends date paid		Jun. 01, 2011	Mar. 01, 2011
Common share dividends date of record		Apr. 29, 2011	Feb. 15, 2011
Common shares, authorized	400,000,000			400,000,000			224,000,000	224,000,000						400,000,000
Total shares, authorized							231,000,000							407,000,000
Preferred stock, shares included in amendment												3,000,000	4,000,000