Stock-Based Compensation (Details) - USD ($)		12 Months Ended						24 Months Ended	36 Months Ended
		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		$ (1,012,000)		$ (861,000)		$ (765,000)
Allocated Share-based Compensation Expense, Net of Tax		1,661,000		1,448,000		1,111,000
Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount		$ 661,000		$ 597,000		$ 331,000
Long Term Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized		850,000						850,000	850,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant, Any Award Type		186,979						186,979	186,979
Share-based Compensation Arrangement By Share Based Payment Award, Number of Shares Available For Grant Stock Options		250,000						250,000	250,000
Option Shares
Balance outstanding, end of period		0						0	0
Number of RSUs [Roll Forward]
Nonvested, end of period		0						0	0
Long Term Incentive Plan [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement by Share Based Payment Award Performance Period		3 years
Share Based Compensation Award Target Share		117,775						117,775	117,775
Unrecognized compensation expense		$ 2,300,000						$ 2,300,000	$ 2,300,000
Unrecognized compensation cost, period for recognition		through 2017
Allocated Share-based Compensation Expense		$ 1,797,000
Weighted Average Price Per LTIP
Nonvested, beginning of period		$ 42.06
Granted		51.78
Vested		46.64
Nonvested, end of period		$ 49.09		$ 42.06				$ 49.09	$ 49.09	$ 42.06
Long Term Incentive Plan [Member] | Performance Shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		235,550						235,550	235,550
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2011-2013 Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted	[1]					9,819
Allocated Share-based Compensation Expense						$ 390,000	[2]				$ 960,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2012-2014 Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted	[1]			8,621
Allocated Share-based Compensation Expense				$ 582,000	[2]					$ 1,821,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2013-2015 Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Granted	[1],[3]	8,465
Share Based Compensation Award Target Share		34,100						34,100	34,100
Allocated Share-based Compensation Expense	[3]	$ 312,000	[2]						$ 1,240,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2013-2015 Grant | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		68,200						68,200	68,200
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2014-2016 Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		39,725						39,725	39,725
Allocated Share-based Compensation Expense		$ 632,000						$ 1,250,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2014-2016 Grant | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		79,450						79,450	79,450
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2015-2017 Grant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		43,950						43,950	43,950
Allocated Share-based Compensation Expense		$ 853,000
Long Term Incentive Plan [Member] | Performance Shares [Member] | 2015-2017 Grant | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Award Target Share		87,900						87,900	87,900
Long Term Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense		$ 2,600,000						$ 2,600,000	$ 2,600,000
Unrecognized compensation cost, period for recognition		through 2019
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period		4 years
Allocated Share-based Compensation Expense		$ 1,300,000		$ 900,000
Number of RSUs [Roll Forward]
Nonvested, beginning of period		70,794		44,567		24,864		44,567	24,864
Granted		37,264		38,765		25,748
Vested		(19,003)		(12,060)		(5,455)
Forfeit		(468)		(478)		(590)
Nonvested, end of period		88,587		70,794		44,567		88,587	88,587	70,794	44,567
Weighted Average Price Per LTIP
Nonvested, beginning of period		$ 44.00		$ 46.27		$ 47.57		$ 46.27	$ 47.57
Granted		46.29		42.19		45.38
Vested		44.81		46.52		48.01
Forfeited		44.99		45.47		46.58
Nonvested, end of period		$ 44.78		$ 44.00		$ 46.27		$ 44.78	$ 44.78	$ 44.00	$ 46.27
Restated Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Cash Received from Exercise of Stock Options		$ 2,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period		10 years 7 days
Share Based Compensation Arrangement By Share Based Payment Award Exchange Period		6 months
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value		$ 200,000		$ 400,000		$ 500,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term		3 years 7 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term		3 years 7 months 12 days
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		$ (118,000)
Option Shares
Balance outstanding, beginning of period		416,088		492,150		529,925		492,150	529,925
Exercised		(62,900)		(69,662)		(33,800)
Forfeited		(500)		(6,400)		(3,975)
Balance outstanding, end of period		352,688		416,088		492,150		352,688	352,688	416,088	492,150
Other Option disclosures
Balance exercisable, end of period		352,688						352,688	352,688
Weighted-average price per share exercisable, end of period		$ 44.00						$ 44.00	$ 44.00
Intrinsic value balance, end of period, Exercisable		$ 2,300,000						$ 2,300,000	$ 2,300,000
Weighted-Average Price Per Share
Balance outstanding, beginning of period		$ 43.40		$ 42.89		$ 42.22		$ 42.89	$ 42.22
Exercised		39.96		39.82		32.16
Forfeited		45.74		43.59		43.72
Balance outstanding, end of period		$ 44.00		$ 43.40		$ 42.89		$ 44.00	$ 44.00	$ 43.40	$ 42.89
Intrinsic Value balance, beginning of period		$ 2,700,000		$ 600,000		$ 1,300,000		$ 600,000	$ 1,300,000
Intrinsic value, Exercised		500,000		500,000		300,000
Intrinsic Value balance, end of period		$ 2,300,000		2,700,000		600,000		$ 2,300,000	$ 2,300,000	$ 2,700,000	$ 600,000
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percent Of Offering Date Price Employees Are Allowed To Purchase Stock		85.00%
Maximum Value Of Stock Employees Are Allowed To Purchase		$ 21,227
Operations and Maintenance Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated Share-based Compensation Expense		$ 2,673,000		$ 2,309,000		$ 1,876,000

[1] In addition to common stock shares, a participant also receives a dividend equivalent cash payment equal to the number of shares of common stock received on the award payout multiplied by the aggregate cash dividends paid per share during the performance period.

[2] Amount represents the expense recognized in the third year of the vesting period noted above.

[3] This represents the estimated number of shares to be awarded as of December 31, 2015 as certain performance share measures had been achieved. Amounts are subject to change with final payout amounts authorized by the Board of Directors in February 2016.