Income Taxes (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Internal Revenue Service (IRS)	Sep. 30, 2012 Internal Revenue Service (IRS)	Dec. 31, 2013 Internal Revenue Service (IRS)	Dec. 31, 2012 Internal Revenue Service (IRS)	Mar. 31, 2013 State and Local Jurisdiction	Dec. 31, 2013 State and Local Jurisdiction	Dec. 31, 2012 State and Local Jurisdiction
Tax Audits [Abstract]
Year(s) no longer subject to audit as statute of limitations has expired						2008
Earliest year subject to examination						2009			2009
Year(s) under examination					2010 and 2011			2009 through 2011
Tax Adjustments, Settlements, and Unusual Provisions				$ 10,000,000
Operating Loss Carryforwards				46,800,000		46,800,000	44,300,000		6,300,000	3,400,000
Tax Credit Carryforward, Amount				4,400,000		4,400,000	8,000,000
Carryforward expiration date range, low						2021			2022
Carryforward expiration date range, high						2033			2031
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	100,000	100,000
Unrecognized tax benefit — Temporary tax positions	1,400,000	1,200,000
Total unrecognized tax benefit	1,500,000	1,300,000	1,500,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1	1,300,000	1,500,000	1,900,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	700,000	500,000	600,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0	(200,000)	(100,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions	500,000	300,000	700,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions	0	(800,000)	(300,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(1,000,000)	0	(1,200,000)
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	0	0	(100,000)
Balance at Dec. 31	1,500,000	1,300,000	1,500,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards	(400,000)	(900,000)
Amounts accrued for penalties related to unrecognized tax benefits	0	0	0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	5.00%	3.40%	4.40%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	(0.90%)	(0.90%)	(0.90%)
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(0.90%)	(0.30%)	0.50%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits	0.00%	0.10%	(0.20%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	(0.20%)	(0.10%)	0.90%
Effective Income Tax Rate Reconciliation, Percent	38.00%	37.20%	39.70%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	5,902,000	930,000	(1,540,000)
Current State and Local Tax Expense (Benefit)	4,628,000	2,216,000	1,573,000
Current Change In Unrecognized Tax Expense (Benefit)	754,000	(69,000)	(1,418,000)
Deferred Federal Income Tax Expense (Benefit)	23,794,000	25,089,000	30,251,000
Deferred State and Local Income Tax Expense (Benefit)	2,720,000	1,890,000	4,105,000
Deferred Change In Unrecognized Tax Expense (Benefit)	(725,000)	128,000	1,254,000
Deferred investment tax credits	(664,000)	(626,000)	(611,000)
Income Tax Expense (Benefit)	36,409,000	29,558,000	33,614,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items	27,516,000	27,995,000	34,017,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(1,676,000)	(837,000)	1,644,000
Other Comprehensive Income (Loss), Tax	(51,000)	(51,000)	(51,000)
Deferred Income Tax Expense (Benefit)	25,789,000	27,107,000	35,610,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	287,121,000	265,202,000
Deferred Tax Liabilities, Regulatory Assets	57,296,000	52,898,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits	16,953,000	16,862,000
Deferred Tax Liabilities, Other	10,193,000	10,535,000
Deferred Tax Liabilities, Net	371,563,000	345,497,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets Environmental Remediation	43,501,000	43,344,000
Deferred Tax Assets, Operating Loss Carryforwards	17,384,000	17,588,000
Deferred Tax Assets Regulatory Liabilities	6,205,000	5,927,000
Deferred Tax Assets Deferred Investment Tax Credits	5,976,000	5,766,000
Deferred Tax Assets Tax credit carryforward	4,440,000	8,011,000
Deferred Tax Assets, Other	3,871,000	2,191,000
Deferred Tax Assets, Net of Valuation Allowance	81,377,000	82,827,000
Deferred Tax Assets, Net	$ 290,186,000	$ 262,670,000