Stock Plans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock And Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, in years		3 years
Pre-tax compensation expense		$ 49	$ 44	$ 51
Deferred tax benefit		18	17	20
Unamortized compensation expense		$ 54
Weighted-average period of recognition, in years		2 years
Weighted-average grant date fair value		$ 46	$ 56	$ 65
Weighted-average grant date fair value, per share (in USD per share)		$ 71.05	$ 59.38	$ 54.54
Total fair value vested		$ 95	$ 78	$ 85
Granted, number of shares (in shares)		641,263
Number of shares remaining (in shares)		2,384,501	3,245,534
Grant date fair value (in USD per share)		$ 60.40	$ 48.45
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period, in years	3 years
Pre-tax compensation expense		$ 6
Unamortized compensation expense		$ 7
Granted, number of shares (in shares)	187,886		0	0
Number of shares remaining (in shares)		170,755
Grant date fair value (in USD per share)		$ 70.39
Common Stock [Member] | 2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares issuable under the stock compensation plan (in shares)		40,000,000
Shares available to be granted (in shares)		38,161,242
Common Stock [Member] | 2015 Directors Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares issuable under the stock compensation plan (in shares)		1,000,000
Shares available to be granted (in shares)		920,942