Share Capital - Narrative (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Shareholders Equity Disclosure [Line Items]
Stock Repurchased and Retired During Period, Shares		12.9
Stock Repurchased and Retired During Period, Value		$ 373,300,000
Stock Repurchase Program, Stock repurchased during period average price per share		$ 29.05
2012 Repurchase Authorization
Shareholders Equity Disclosure [Line Items]
Stock Repurchase Program, Authorized Amount		750,000,000
Stock Repurchased and Retired During Period, Shares		3.8	18.3
Stock Repurchased and Retired During Period, Value		98,300,000	401,600,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	250,000,000
2013 Repurchase Authorization
Shareholders Equity Disclosure [Line Items]
Stock Repurchase Program, Authorized Amount		850,000,000
Stock Repurchased and Retired During Period, Shares	4.8
Stock Repurchased and Retired During Period, Value	150,000,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount		575,000,000
Stock Repurchase Program, Stock repurchased during period average price per share	$ 30.97
Stock options
Shareholders Equity Disclosure [Line Items]
Weighted Average Grant Date Fair Value Per Stock Option		$ 8.34
Number of Stock Options Granted During the Period		1.0
Restricted stock awards
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period		0.2
Aggregate Grant Date Fair Value of Award		5,300,000
Restricted stock units
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period		1.7
Aggregate Grant Date Fair Value of Award		48,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Performance units
Shareholders Equity Disclosure [Line Items]
Number of Awards Granted During the Period		0.7
Aggregate Grant Date Fair Value of Award		$ 17,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		3 years
Share Based Compensation Arrangement Performance Units Maximum Share Payout Number		1.4
Minimum [Member] | Performance units
Shareholders Equity Disclosure [Line Items]
Performance Units Final Payout Percentage Of Grant Amount		0.00%
Maximum [Member] | Performance units
Shareholders Equity Disclosure [Line Items]
Performance Units Final Payout Percentage Of Grant Amount		200.00%