Stock-Based Compensation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 21,239
LTIP Paragraph 1 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	850,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	225,669
Additional Ltip Shares Authorized	250,000
LTIP Paragraph 2 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 42.06	$ 43.39
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value	$ 43.67
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 42.43
LTIP unrecognized compensation expense	1,700,000
LTIP Table 1 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period	8,408	9,819	9,924
Allocated Share-based Compensation Expense	600,000	400,000	500,000
Cumulative LTIP Expense Evaluation Period Vesting In Current Year	1,821,000
Cumulative LTIP Expense Evaluation Period Vested In Prior Year	1,000,000
Cumulative LTIP Expense Evaluation Period Vested Two Years Ago	1,200,000
LTIP Table 2 [Abstract]
Target LTIP Award For Period Vesting In Current Year	35,340,000
Target LTIP Award For Period Vesting Next Year	37,300,000
Target LTIP Award For Period Vesting In Two Years	43,625,000
Target LTIP Award For All Periods	116,265,000
Maximum LTIP Award For Period Vesting In Current Year	70,680,000
Maximum LTIP Award For Period Vesting Next Year	74,600,000
Maximum LTIP Award For Period Vesting In Two Years	87,250,000
Maximum LTIP Award For All Periods	232,530,000
Current Year Expense LTIP Award For Period Vesting In Current Year	583,000
Current Year Expense LTIP Award For Period Vesting Next Year	442,000
Current Year Expense LTIP Award For Period Vesting In Two Years	618,000
Current Year Expense LTIP Award For All Periods	1,643,000
Cumulative LTIP Expense Evaluation Period Vesting In Current Year	1,821,000
Cumulative Expense LTIP Award For Period Vesting Next Year	928,000
Cumulative Expense LTIP Award For Period Vesting In Two Years	618,000
RSU Paragraph 1 [Abstract]
Unrecognized compensation cost, restricted stock units, period for recognition	through 2019
RSU Table 1 [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	70,794	44,567	24,864	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	38,765	25,748	25,224
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	(12,060)	(5,455)	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(478)	(590)	(360)
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Nonvested, Weighted Average Grant Date Fair Value	$ 44.00	$ 46.27	$ 47.57	$ 0.00
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Grants in Period, Weighted Average Grant Date Fair Value	$ 42.19	$ 45.38	$ 47.58
Share-based Compensation Arrangement by Share-based Payment Award, Restricted Stock Units, Vested in Period, Weighted Average Grant Date Fair Value	$ 46.52	$ 48.01	$ 0.00
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value	$ 45.47	$ 46.58	$ 48.00
SOP Paragraph 1 [Abstract]
Reserved Shares Stock Options	416,088	492,150	529,925	579,225
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	10 years 7 days
SOP Paragraph 2 [Abstract]
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options	2,800,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	(117,000)	(242,000)	(149,000)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Total Fair Value	400,000	500,000	600,000
Unrecognized compensation cost, period for recognition	through 2016
SOP Table 2 [Abstract]
RSU compensation expense	900,000	600,000
RSU unrecognized compensation expense	2,200,000
Reserved Shares Stock Options	416,088	492,150	529,925	579,225
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	(6,400)	(3,975)	(2,475)
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Number of Exercisable Options	388,965
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Exercise Price	$ 43.40	$ 42.89	$ 42.22	$ 42.09
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 39.82	$ 32.16	$ 40.62
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 43.59	$ 43.72	$ 43.78
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value	500,000	300,000	400,000
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Exercisable Options, Weighted Average Exercise Price	$ 43.23
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	2,700,000	600,000	1,300,000	3,400,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(69,662)	(33,800)	(46,825)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	2,600,000
Stock Based Compensation FS Impact Summary Table [Abstract]
Stock Based Compensation Charged To Operations And Maintenance	2,309,000	1,876,000	1,668,000
Income Tax Benefit Of Stock Based Compensation	(861,000)	(765,000)	(707,000)
Net Income Impact Of Stock Based Compensation	1,448,000	1,111,000	961,000
Stock Based Compensation Capitalized	$ 597,000	$ 331,000	$ 294,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term	4 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	4 years 3 months 18 days