Share Transactions - Share Repurchases (Details) (Common Shares, USD $) Share data in Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
Common Shares
Class of Stock [Line Items]
Cumulative number of shares acquired since inception of share repurchase program		109.9
Aggregate purchase price of shares acquired since inception of share repurchase program		$ 2,790,000,000
Treasury stock, shares acquired (shares)	0.3	1.2
Treasury stock, value of shares acquired	15,500,000	56,500,000
Remaining authorized repurchase amount		$ 713,400,000