Equity (Assumptions and Options) (Details) - Options [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Award maximum term	10 years
Assumptions used to estimate the fair value of options using the Black-Scholes option pricing model [Abstract]
Expected volatility (in hundredths)	14.91%	16.37%	18.91%
Expected dividends (in hundredths)	3.16%	3.16%	3.11%
Expected term (years)	7 years	7 years	7 years
Risk-free rate (in hundredths)	2.23%	1.50%	1.84%
Option activity [Roll Forward]
Shares underlying options - Balance at beginning of year (in shares)	2,505,208
Shares underlying options - Granted (in shares)	407,216
Shares underlying options - Exercised (in shares)	(429,402)
Shares underlying options - Balance at end of year (in shares)	2,483,022	2,505,208
Shares underlying options - Exercisable (in shares)	1,825,151
Additional disclosures pertaining to options [Abstract]
Balance at beginning of year, weighted average exercise price (in dollars per share)	$ 71.08
Granted, weighted average exercise price (in dollars per share)	126.86
Exercised, weighted average exercise price (in dollars per share)	52.47
Balance at end of year, weighted average exercise price (in dollars per share)	83.45	$ 71.08
Exercisable at end of year, weighted average exercise price (in dollars per share)	$ 70.17
Balance at end of year, weighted average remaining contractual term (years)	5 years 7 months 6 days
Exercisable at end of year, weighted average remaining contractual term (years)	4 years 6 months
Balance at end of year, aggregate intrinsic value	$ 0
Exercisable at end of year, aggregate intrinsic value	$ 0
Granted, weighted average grant date fair value (in dollars per share)	$ 13.25	$ 11.74	$ 13.62
Total intrinsic value of stock options exercised	$ 41	$ 42	$ 11
Cash received from option exercises	23	36	9
Tax benefit realized from options exercised	$ 16	$ 16	$ 4