Income Taxes (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	12 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)	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		2008
Year(s) under examination					2010 and 2011
Tax Adjustments, Settlements, and Unusual Provisions				$ 10,000,000
Operating Loss Carryforwards				168,700,000		168,700,000	158,000,000	23,600,000	21,000,000
Tax Credit Carryforward, Amount				1,700,000		1,700,000	1,300,000
Carryforward expiration date range, low						2021		2014
Carryforward expiration date range, high						2033		2031
Unrecognized Tax Benefits [Abstract]
Unrecognized tax benefit — Permanent tax positions	1,200,000	200,000
Unrecognized tax benefit — Temporary tax positions	2,900,000	3,700,000
Total unrecognized tax benefit	4,100,000	3,900,000	4,800,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at Jan. 1	3,900,000	4,800,000	4,300,000
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions	1,600,000	1,100,000	1,500,000
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	0	(1,600,000)	(200,000)
Unrecognized Tax Benefits Increases Resulting From Prior Period Tax Positions	3,100,000	800,000	2,500,000
Unrecognized Tax Benefits Decreases Resulting From Prior Period Tax Positions	(300,000)	(1,200,000)	(300,000)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	(4,200,000)	0	(3,000,000)
Balance at Dec. 31	4,100,000	3,900,000	4,800,000
Tax Benefits Associated With NOL And Tax Credit Carryforwards [Abstract]
NOL and tax credit carryforwards	(2,400,000)	(2,000,000)
Upper bound of decrease in unrecognized tax benefit that is reasonably possible	(2,000,000)
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	2.00%	2.20%	2.90%
Effective Income Tax Rate Reconciliation Change In Unrecognized Tax Benefits	0.70%	0.00%	0.00%
Effective Income Tax Rate Reconciliation Regulatory Differences Utility Plant Items	(1.10%)	(0.40%)	(0.10%)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	(0.40%)	(0.20%)	(0.30%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	(0.10%)	0.30%	0.50%
Effective Income Tax Rate Reconciliation, Percent	36.10%	36.90%	38.00%
Components of Income Tax Expense (Benefit), Continuing Operations [Abstract]
Current Federal Tax Expense (Benefit)	14,947,000	6,549,000	(1,993,000)
Current State and Local Tax Expense (Benefit)	2,943,000	2,712,000	3,287,000
Current Change In Unrecognized Tax Expense (Benefit)	(263,000)	(824,000)	(1,394,000)
Deferred Federal Income Tax Expense (Benefit)	33,489,000	50,189,000	50,903,000
Deferred State and Local Income Tax Expense (Benefit)	1,754,000	3,069,000	3,240,000
Deferred Change In Unrecognized Tax Expense (Benefit)	1,232,000	861,000	1,365,000
Deferred investment tax credits	(341,000)	(327,000)	(275,000)
Income Tax Expense (Benefit)	53,761,000	62,229,000	55,133,000
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Deferred tax expense (benefit) excluding selected items	38,333,000	55,749,000	56,181,000
Amortization and adjustments to deferred income taxes on income tax regulatory assets and liabilities	(1,761,000)	(1,533,000)	(575,000)
Other Comprehensive Income (Loss), Tax	(97,000)	(97,000)	(98,000)
Deferred Income Tax Expense (Benefit)	36,475,000	54,119,000	55,508,000
Deferred Tax Liabilities, Gross [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	705,416,000	654,259,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Employee Benefits	52,081,000	51,394,000
Deferred Tax Liabilities, Other	30,066,000	24,034,000
Deferred Tax Liabilities, Net	787,563,000	729,687,000
Deferred Tax Assets, Gross [Abstract]
Deferred Tax Assets, Operating Loss Carryforwards	61,330,000	59,562,000
Deferred Tax Assets Rate Refund	17,192,000	2,911,000
Deferred Tax Assets Unbilled Revenue Fuel Costs	13,316,000	10,555,000
Deferred Tax Assets Regulatory Liabilities	9,724,000	6,176,000
Deferred Tax Assets, Tax Deferred Expense, Deferred Fuel Costs	6,877,000	11,952,000
Deferred Tax Assets, Other	6,708,000	4,448,000
Deferred Tax Assets, Net of Valuation Allowance	115,147,000	95,604,000
Deferred Tax Assets, Net	$ 672,416,000	$ 634,083,000