Stock-Based Compensation (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
ESPP Paragraph [Abstract]
Percent Of Offering Date Price Employees Are Allowed To Purchase Stock	85.00%
Maximum Value Of Stock Employees Are Allowed To Purchase	$ 21,244
LTIP Paragraph 1 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	850,000	600,000
Additional Ltip Shares Authorized	250,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	311,571
Cumulative LTIP Expense Evaluation Period Vested In Prior Year	800,000
Cumulative LTIP Expense Evaluation Period Vested Two Years Ago	700,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	9,022	8,428	8,007
Allocated Share-based Compensation Expense	400,000	200,000
LTIP Paragraph 2 [Abstract]
Weighted-average per share grant date fair value of unvested shares	$ 51.42	$ 25.06
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value	$ 45.05
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 22.35
Shares Granted Under RSU	25,224
Stock Price On Restricted Stock Unit Grant Date, Minimum	$ 44.43
Stock Price On Restricted Stock Unit Grant Date, Maximum	$ 48.25
LTIP Table 1 [Abstract]
Target LTIP Award For Period Vesting In Current Year	41,500,000
Target LTIP Award For Period Vesting Next Year	37,950,000
Target LTIP Award For Period Vesting In Two Years	35,340,000
Target LTIP Award For All Periods	114,790,000
Maximum LTIP Award For Period Vesting In Current Year	83,000,000
Maximum LTIP Award For Period Vesting Next Year	75,900,000
Maximum LTIP Award For Period Vesting In Two Years	70,680,000
Maximum LTIP Award For All Periods	229,580,000
Current Year Expense LTIP Award For Period Vesting In Current Year	452,000
Current Year Expense LTIP Award For Period Vesting Next Year	294,000
Current Year Expense LTIP Award For Period Vesting In Two Years	635,000
Current Year Expense LTIP Award For All Periods	1,381,000
Cumulative Expense LTIP Award For Period Vesting In Current Year	1,170,000
Cumulative Expense LTIP Award For Period Vesting Next Year	570,000
Cumulative Expense LTIP Award For Period Vesting In Two Years	635,000
SOP Paragraph 1 [Abstract]
Reserved Shares Stock Options	529,925	579,225	490,460	484,935
Unrecognized compensation cost, period for recognition	10 years 7 days
Option Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Method Used	Black-Scholes
SOP Paragraph 2 [Abstract]
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options	700,000
Employee Service Share-based Compensation, Tax Benefit Realized from Exercise of Stock Options	100,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value	600,000	600,000	500,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Remaining Contractual Term	5 years 1 month 5 days
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term	5 years 9 months 17 days
Unrecognized compensation cost	500,000
Unrecognized compensation cost, period for recognition	through 2014
SOP Table 1 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate		2.00%	2.30%
Option Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Term		4.5	4.7
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate		24.50%	23.20%
Option Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Expected Dividend Rate		3.80%	3.80%
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Forfeiture Rate		3.10%	3.20%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value		$ 6.73	$ 6.36
SOP Table 2 [Abstract]
Reserved Shares Stock Options	529,925	579,225	490,460	484,935
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross		122,700	119,750
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(46,825)	(24,185)	(111,525)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	(2,475)	(9,750)	(2,700)
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options	366,887
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price	$ 42.22	$ 42.09	$ 40.82	$ 39.57
Stock Price On Stock Option Plan Grant Date		$ 45.74	$ 44.25
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 40.62	$ 33.88	$ 39.01
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 43.78	$ 44.38	$ 43.00
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	400,000	300,000	900,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price	$ 41.16
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	1,300,000	3,400,000	2,800,000	2,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	1,200,000
Stock Based Compensation FS Impact Summary Table [Abstract]
Stock Based Compensation Charged To Operations And Maintenance	1,668,000	1,477,000	1,032,000
Income Tax Benefit Of Stock Based Compensation	(707,000)	(597,000)	(418,000)
Net Income Impact Of Stock Based Compensation	961,000	880,000	614,000
Stock Based Compensation Capitalized	$ 294,000	$ 261,000	$ 182,000