Repurchases of Outstanding Securities (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Accelerated Stock Buyback (Details) [Abstract]
Accelerated stock buyback, gross price paid	$ 200,000,000
Accelerated stock buyback, initial shares delivered	4,592,423
Accelerated stock buyback, value of initial shares delivered	160,000,000
Accelerated stock buyback, final settlement expected	40,000,000
Repurchases of Outstanding Securities (Details) [Abstract]
Authorized aggregate amount	1,250,000,000
Total number of shares repurchased since inception of our stock repurchase program (shares)	36,177,737	31,120,279
Aggregate purchase price of shares repurchased since inception of our stock repurchase program	$ 994,800,000	$ 777,300,000