Business Combinations (Details 3) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Summary of unaudited consolidated pro forma results of operations
Revenues	$ 13,443	$ 12,077
Income from continuing operations	198	186
Net income	$ 189	$ 187
Income from continuing operations per diluted common share (in dollars per share)	$ 2.18	$ 2.03
Net income per diluted common share (in dollars per share)	$ 2.09	$ 2.05

The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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