Common Shareholders' Equity (Assumptions and Options) (Details) (Options [Member], USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Options [Member]
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)	20.32%	[1]			21.00%	[1]
Expected volatility, low range (in hundredths)			20.08%	[1]
Expected volatility, high range (in hundredths)			20.15%	[1]
Expected dividends (in hundredths)	3.11%				3.99%
Expected dividends, low range (in hundredths)			3.28%
Expected dividends, high range (in hundredths)			3.64%
Expected term (years)	7 years	[2]	7 years	[2]	6 years 8 months 12 days	[2]
Risk-free rate (in hundredths)	2.17%				1.37%
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)	3,191,547
Shares underlying options - Granted (in shares)	198,358
Shares underlying options - Exercised (in shares)	(564,870)
Shares underlying options - Forfeited (in shares)	0
Shares underlying options - Expired (in shares)	0
Shares underlying options - Balance at end of year (in shares)	2,825,035		3,191,547
Shares underlying options - Exercisable (in shares)	2,344,937
Additional disclosures pertaining to options [Abstract]
Balance at beginning of year, weighted average exercise price (in dollars per share)	$ 54.70
Granted, weighted average exercise price (in dollars per share)	$ 93.27
Exercised, weighted average exercise price (in dollars per share)	$ 46.51
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)	$ 59.04		$ 54.70
Exercisable at end of year, weighted average exercise price (in dollars per share)	$ 55.08
Balance at end of year, weighted average remaining contractual term (years)	5 years 9 months 18 days
Balance at end of year, aggregate intrinsic value	$ 133
Exercisable at end of year, weighted average remaining contractual term (years)	5 years 2 months 12 days
Exercisable at end of year, aggregate intrinsic value	120
Granted, weighted average grant date fair value (in dollars per share)	$ 14.09		$ 9.20		$ 7.69
Total intrinsic value of stock options exercised	30		14		57
Cash received from option exercises	26		14		55
Tax benefit realized from options exercised	$ 11		$ 5		$ 22

[1] Based on historical experience.

[2] Based on historical exercise and post-vesting cancellation experience adjusted for outstanding awards.