Share-Based Compensation Plans - Employee Share Purchase Plan Narrative (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 328	$ 300	$ 212
United States [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of interval for purchase of common stock			6 months	3 months
Purchase price expressed as a percentage of the fair market value of common stock			85.00%	85.00%
Stock issued to employees under the plan (in shares)	439,000	556,000	621,000
Total share-based compensation expense	7	6	6
United States [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of shares under plan	7,500,000
United Kingdom [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of shares under plan	300,000	350,000		300,000
Stock issued to employees under the plan (in shares)	642	172,000	25,000
Waiting period before purchase of shares	3 years
Total share-based compensation expense	$ 2	$ 1	$ 1