Share Capital (Narrative) (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Stockholders' Equity Attributable to Parent [Abstract]
Shares repurchased	3.2	11.3	6.3	13.4
Cost of repurchased shares	$ 75,000,000	$ 279,900,000	$ 150,000,000	$ 333,000,000
Shares withheld to meet employees' tax withholding obligations	1.7	2.7	1.7	2.7
Fair Value Of Shares Withheld			42,600,000	70,800,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount			582,000,000	835,400,000
Treasury stock shares	55.8	49.5	55.8	49.5
Treasury shares held, as unvested restricted stock awards	10.6	10.0	10.6	10.0
Common shares market price (per share)			$ 22.60
Treasury shares market value			$ 1,300,000,000