Stock-Based Compensation (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
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	$ 21,210
LTIP Paragraph 2 [Abstract]
Weighted-average per share grant date fair value of unvested shares	$ 25.06	$ 23.1
Weighted-average per share grant date fair value of vested shares	$ 22.35
Weighted-average per share grant date fair value of granted shares	$ 19.38
LTIP Table 1 [Abstract]
Threshold For 2009 To 2011 LTIP Period	7,410
Target LTIP Award For Period Vesting In Current Year	39,000
Target LTIP Award For Period Vesting Next Year	41,500
Target LTIP Award For Period Vesting In Two Years	37,950
Target LTIP Award For All Periods	118,450
Maximum LTIP Award For Period Vesting In Current Year	78,000
Maximum LTIP Award For Period Vesting Next Year	83,000
Maximum LTIP Award For Period Vesting In Two Years	75,900
Maximum LTIP Award For All Periods	236,900
Current Year Expense LTIP Award For Period Vesting In Current Year	353,000
Current Year Expense LTIP Award For Period Vesting Next Year	430,000
Current Year Expense LTIP Award For Period Vesting In Two Years	276,000
Current Year Expense LTIP Award For All Periods	1,059,000
Cumulative Expense LTIP Award For Period Vesting In Current Year	763,000
Cumulative Expense LTIP Award For Period Vesting Next Year	718,000
Cumulative Expense LTIP Award For Period Vesting In Two Years	276,000
Share Based Compensation Arrangement By Share Based Payment Award Additional General Disclosures Abstract
LTIP Shares Authorized	600,000
LTIP Shares Available for Award	337,788
LTIP Vesting Period	three-year performance periods
Awarded LTIP shares (or estimated at yearend)	8,428	8,257	15,900
LTIP expense		200,000	500,000
Cumulative LTIP Expense Evaluation Period Vested In Prior Year	700,000
Cumulative LTIP Expense Evaluation Period Vested Two Years Ago	1,500,000
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures Abstract
SOP shares authorized	2,400,000
SOP shares available for grant	580,650
SOP Vesting	up to 10 years and 7 days
Fair value pricing model SOP	Black-Scholes
SOP Paragraph 2 [Abstract]
Cash received for options exercised	800,000
Tax benefit for cash received for options exercised	26,000
Fair value of options vested	600,000	500,000	400,000
Weighted average remaining life of exercisable options	5.5
Outstanding average remaining life in years	6.8
Unrecognized compensation cost	1,000,000
Unrecognized compensation cost, period for recognition	2014
SOP Table 1 [Abstract]
Risk-free interest rate	2.00%	2.30%	2.00%
Expected life (in years)	4.5	4.7	4.7
Expected market price volatility factor	24.50%	23.20%	22.50%
Expected dividend yield	3.80%	3.80%	3.80%
Forfeiture rate	3.10%	3.20%	3.70%
Weighted-average grant date fair value of SOP shares granted	$ 6.73	$ 6.36	$ 5.46
SOP Table 2 [Abstract]
Number of Outstanding Options	579,225	490,460	484,935	396,410
Shares granted under SOP	122,700	119,750	111,750
Number of Options Exercised	(24,185)	(111,525)	(23,225)
Number of Options Forfeited	(9,750)	(2,700)	0
Weighted average option price at balance sheet date	$ 42.09	$ 40.82	$ 39.57	$ 38.62
Stock Price On Stock Option Plan Grant Date	$ 45.74	$ 44.25	$ 41.15
Weighted average price of options exercised	$ 33.88	$ 39.01	$ 30.92
Weighted average price of options forfeited	$ 44.38	$ 43.00
Intrinsic Value of options outstanding	3,400,000	2,800,000	2,700,000	2,300,000
Intrinsic Value of options exercised	300,000	900,000	300,000
Exercisable stock options	311,951
Weighted average exercise prices of exercisable options	$ 40.20
Aggregate intrinsic value of exercisable options	2,400,000
Stock Based Compensation FS Impact Summary Table [Abstract]
Stock Based Compensation Charged To Operations And Maintenance	1,477,000	1,032,000	1,434,000
Income Tax Benefit Of Stock Based Compensation	(597,000)	(418,000)	(559,000)
Net Income Impact Of Stock Based Compensation	880,000	614,000	875,000
Stock Based Compensation Capitalized	$ 261,000	$ 182,000	$ 229,000