Capital transactions (Details) (USD $)	9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Feb. 03, 2011	Sep. 30, 2012 2011 Stock Repurchase Program	Sep. 30, 2012 New Share Repurchase Program	Feb. 02, 2012 New Share Repurchase Program
Share Repurchase Program [Line Items]
Share repurchase, cumulative amount authorized			$ 1,500,000,000			$ 1,000,000,000
Shares of common stock agreed to repurchase	$ 315,000,000	$ 524,000,000		$ 261,000,000	$ 54,000,000
Shares of common stock repurchased	26,000,000	47,000,000		22,000,000	4,000,000