Share Capital (Details) - Authorized and issued (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Common Class A [Member] 2010 Repurchase Authorization [Member]	Mar. 31, 2012 Common Class A [Member] 2012 Repurchase Authorization [Member]	Jun. 30, 2012 Common Class A [Member] 2012 Repurchase Authorization [Member]
Equity Class Of Treasury Stock [Line Items]
Stock Repurchase Program Authorized Amount		$ 190.0		$ 750.0
Stock Repurchase Program Remaining Authorized Repurchase Amount				525.0
Treasury Stock Value Retired Par Value Method	$ 125.0		$ 100.0
Treasury Stock, Shares, Retired (in Shares)	6.1		4.7