Share-Based Compensation Plans - Employee Share Purchase Plan Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total share-based compensation expense	$ 339	$ 328	$ 300
United States [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Period of interval for purchase of common stock (in months)			6 months
Purchase price expressed as a percentage of the fair market value of common stock (in percentage)			85.00%
Stock issued to employees under the plan (in shares)	411,636	439,000	556,000
Total share-based compensation expense	$ 9	$ 7	$ 6
United States [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of shares under plan (in shares)	7,500,000.0
United Kingdom [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Purchase of shares under plan (in shares)	100,000	300,000	350,000
Stock issued to employees under the plan (in shares)	2,779	642	172,110
Waiting period before purchase of shares (in years)	3 years
Total share-based compensation expense	$ 2	$ 1	$ 1