Capital transactions (Details) (USD $)	6 Months Ended	1 Months Ended		6 Months Ended
	Jun. 30, 2011	Jan. 31, 2011 Share Repurchase Program	Dec. 31, 2010 Share Repurchase Program	Jun. 30, 2011 New Share Repurchase Program	Feb. 03, 2011 New Share Repurchase Program
Share Repurchase Program [Line Items]
Share repurchase, cumulative amount authorized			$ 1,000,000,000		$ 1,500,000,000
Shares of common stock agreed to repurchase	$ 501,000,000	$ 22,000,000		$ 479,000,000
Shares of common stock agreed to repurchase (in shares)		1.8		43