Compensation and Benefit Plans (Details 4) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years		Dec. 31, 2010		Dec. 31, 2009
Share-based Compensation Arrangement by Share-based Payment Award
Shares authorized by new plan or amendments (in shares)	49.5
Shares remaining for future issuance under stock-based compensation plans (in shares)	30
Number of years from the grant of options during which dividend equivalents accrue (in years)	5
Supplemental Data on Stock Based Compensation
Stock Based Compensation Expense	$ 37	[1]	$ 44	[1]	$ 33	[1]
Income tax benefits related to stock compensation expense	15		17		13
Excess tax benefits	13	[2]	8	[2]	9	[2]
Weighted average grant date fair value per option granted (in dollars per share)	$ 5.61		$ 4.89		$ 3.05
Fair value of options vested	18		18		14
Cash used to purchase shares to settle options	90		61		25
Cash from participants to exercise stock options	59		38		13
Value of options exercised	31		23		12
Tax benefit from options exercised	12		9		5
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award
Expiration period following grant date (in years)	10 years
Vesting period (in years)	4 years
Assumptions used in the Black-Scholes option-pricing model
Expected term (in years)	7.0		7.3		7.4
Risk-free interest rate, low end of the range (as a percent)	1.40%		2.00%		2.80%
Risk-free interest rate, high end of the range (as a percent)	3.10%		3.20%		3.50%
Expected dividend yield, low end of the range (as a percent)	3.10%		3.30%		3.60%
Expected dividend yield, high end of the range (as a percent)	3.50%		4.00%		5.00%
Weighted-average expected dividend yield (as a percent)	3.40%		3.80%		4.90%
Expected volatility, low end of the range (as a percent)	18.20%		18.80%		20.00%
Expected volatility, high end of the range (as a percent)	19.00%		19.80%		21.00%
Weighted-average volatility (as a percent)	18.90%		19.80%		20.60%
Historical period used in calculating the expected volatility (in months)	84		87		84
Aggregate Intrinsic Value
Total unrecognized compensation cost	20
Weighted-average period, recognition of unrecognized compensation cost (in years)	2
Supplemental Data on Stock Based Compensation
Stock Based Compensation Expense	16	[1]	18	[1]	13	[1]
Performance Shares
Assumptions used in the Black-Scholes option-pricing model
Historical period used in calculating the expected volatility (in months)	36
Aggregate Intrinsic Value
Total unrecognized compensation cost	4
Weighted-average period, recognition of unrecognized compensation cost (in years)	2
Vesting, period of continuous service (in years)	three-calendar-year period
Vesting, market condition, performance period (in years)	3
Supplemental Data on Stock Based Compensation
Stock Based Compensation Expense	6	[1]	10	[1]	5	[1]
Performance shares classified as equity awards
Aggregate Intrinsic Value
Grant date risk-free interest rate (as a percent)	1.20%		1.30%		1.30%
Grant date expected volatility (as a percent)	20.40%		21.60%		21.40%
Weighted-Average Fair Value
Granted (in dollars per share)	$ 29.97	[3]	$ 32.25	[3]	$ 21.42	[3]
Supplemental Data on Stock Based Compensation
Fair value of shares vested	4	[3]	4	[3]	1	[3]
Performance shares classified as liability awards
Aggregate Intrinsic Value
Grant date expected volatility (as a percent)	15.90%	[4]	20.60%	[4]	21.90%	[4]
2011 Performance shares classified as liability awards
Aggregate Intrinsic Value
Grant date risk-free interest rate (as a percent)	0.30%	[4]	0.00%	[4]	0.00%	[4]
2010 Performance shares classified as liability awards
Aggregate Intrinsic Value
Grant date risk-free interest rate (as a percent)	0.20%	[4]	0.60%	[4]	0.00%	[4]
2009 Performance shares classified as liability awards
Aggregate Intrinsic Value
Grant date risk-free interest rate (as a percent)	0.00%	[4]	0.30%	[4]	1.10%	[4]
Restricted Stock Units
Aggregate Intrinsic Value
Total unrecognized compensation cost	8
Compensation cost to be recognized in 2012	3
Compensation cost to be recognized in 2013	5
Vesting, period of continuous service (in years)	three-calendar-year-plus-two-days vesting period
Weighted-Average Fair Value
Granted (in dollars per share)	$ 38.01		$ 32.12		$ 25.21
Supplemental Data on Stock Based Compensation
Stock Based Compensation Expense	8	[1]	7	[1]	5	[1]
Value of shares settled	6.00		0		0
Tax benefits realized from settlements of awards	3.00		0		0
Other Stock Awards
Supplemental Data on Stock Based Compensation
Stock Based Compensation Expense	$ 7	[1]	$ 9	[1]	$ 10	[1]

[1] Reflected in "Operations and maintenance" on the consolidated statements of income.

[2] Reflected in "Settlements of stock based compensation – net" in the financing section of the consolidated statements of cash flows.

[3] There were no settlements of awards for performance shares in 2011, 2010 and 2009 as performance targets were not met.

[4] The portion of performance shares classified as share-based liability awards are revalued at each reporting period.