Earnings Per Share

12 Months Ended

Dec. 31, 2012

Earnings Per Share [Abstract]

Earnings Per Share

Basic earnings per share is calculated by dividing net income attributable to common shareholders by the weighted average number of shares outstanding during the period, excluding treasury shares. The weighted average number of shares outstanding during the period also includes participating securities such as unvested time-based restricted stock awards and restricted stock units that pay dividend equivalents. Diluted earnings per share is computed using the treasury stock method, which requires computing share equivalents and dividing net income attributable to common shareholders by the total weighted average number of shares and share equivalents outstanding during the period.

The calculation of earnings per share is as follows:

$ in millions, except per share data	Net Income Attributable to Common Shareholders		Weighted Average Number of Shares	Per Share Amount
For the year ended December 31, 2012
Basic earnings per share		$677.1	452.3		$1.50
Dilutive effect of share-based awards	—		1.5	—
Diluted earnings per share		$677.1	453.8		$1.49
For the year ended December 31, 2011
Basic earnings per share		$729.7	462.9		$1.58
Dilutive effect of share-based awards	—		1.8	—
Diluted earnings per share		$729.7	464.7		$1.57
For the year ended December 31, 2010
Basic earnings per share		$465.7	460.4		$1.01
Dilutive effect of share-based awards	—		2.8	—
Diluted earnings per share		$465.7	463.2		$1.01

See Note 12, “Share-Based Compensation,” for a summary of share awards outstanding under the company's share-based payment programs. These programs could result in the issuance of common shares that would affect the measurement of basic and diluted earnings per share.

No share options were excluded from the computation of diluted earnings per share during the year ended December 31, 2012, because the option exercise price was greater than the average market price of the common shares and therefore their inclusion would have been anti-dilutive (December 31, 2011: 0.1 million share options at a weighted average exercise price of £18.11; December 31, 2010: 5.7 million share options at a weighted average exercise price of £19.47).

There were no time-vested share awards that were excluded from the computation of diluted earnings per share during the years ended December 31, 2012, 2011, and 2010 due to their inclusion being anti-dilutive. There were 0.2 million contingently issuable shares excluded from the diluted earnings per share computation during year ended December 31, 2012 (December 31, 2011: none; December 31, 2010: none), because the necessary performance conditions for the shares to be issuable had not yet been satisfied at the end of the respective period.