Common Shareholders' Equity (Assumptions and Options) (Details) - Options [Member] - USD ($) $ / shares in Units, $ 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]
Vesting period (in years)	3 years
Maximum term (in years)	10 years
Assumptions used to estimate the fair value of options using the Black-Scholes option pricing model [Abstract]
Expected volatility (in hundredths)	18.91%	20.32%
Expected volatility, low range (in hundredths)			20.08%
Expected volatility, high range (in hundredths)			20.15%
Expected dividends (in hundredths)	3.11%	3.11%
Expected dividends, low range (in hundredths)			3.28%
Expected dividends, high range (in hundredths)			3.64%
Expected term (years)	7 years	7 years	7 years
Risk-free rate (in hundredths)	1.84%	2.17%
Risk-free rate, low range (in hundredths)			1.15%
Risk-free rate, high range (in hundredths)			1.40%
Option activity [Roll Forward]
Shares underlying options - Balance at beginning of year (in shares)	2,825,035
Shares underlying options - Granted (in shares)	229,158
Shares underlying options - Exercised (in shares)	(187,692)
Shares underlying options - Forfeited (in shares)	0
Shares underlying options - Expired (in shares)	0
Shares underlying options - Balance at end of year (in shares)	2,866,501	2,825,035
Shares underlying options - Exercisable (in shares)	2,415,194
Additional disclosures pertaining to options [Abstract]
Balance at beginning of year, weighted average exercise price (in dollars per share)	$ 59.04
Granted, weighted average exercise price (in dollars per share)	103.62
Exercised, weighted average exercise price (in dollars per share)	47.03
Forfeited, weighted average exercise price (in dollars per share)	0
Expired, weighted average exercise price (in dollars per share)	0
Balance at end of year, weighted average exercise price (in dollars per share)	63.39	$ 59.04
Exercisable at end of year, weighted average exercise price (in dollars per share)	$ 57.62
Balance at end of year, weighted average remaining contractual term (years)	5 years 4 months
Balance at end of year, aggregate intrinsic value	$ 116
Exercisable at end of year, weighted average remaining contractual term (years)	4 years 8 months
Exercisable at end of year, aggregate intrinsic value	$ 112
Granted, weighted average grant date fair value (in dollars per share)	$ 13.62	$ 14.09	$ 9.20
Total intrinsic value of stock options exercised	$ 11	$ 30	$ 14
Cash received from option exercises	9	26	14
Tax benefit realized from options exercised	$ 4	$ 11	$ 5