Income Tax (details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Deferred Tax Assets And Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	$ 292,235,000	$ 255,471,000
Deferred Tax Liabilities Regulatory Taxes Paid	2,106,000	5,272,000
Deferred Tax Liabilities, Regulatory Assets	65,755,000	68,822,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities	35,638,000	23,159,000
Deferred Tax Liabilities Non Regulated Liabilities	43,373,000	34,544,000
Deferred Tax Liabilities	439,107,000	387,268,000
Deferred Tax Assets, Regulatory Assets and Liabilities	4,727,000	1,402,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	5,119,000	4,342,000
Deferred Tax Assets Non Regulated Assets	1,161,000	772,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	1,626,000	1,702,000
Deferred Tax Assets, Tax Credit Carryforwards, Other	14,255,000	7,071,000
Deferred Tax Assets, Net	26,888,000	15,289,000
Deferred Tax Assets (Liabilities), Net	412,219,000	371,979,000
Accumulated Deferred Investment Tax Credit	(990,000)	(1,430,000)
Deferred tax liabilities	413,209,000	373,409,000
Income Tax Disclosure [Abstract]
Federal Refund From Prior Year Tax Return	14,400,000
Federal Refund From Second Prior Year Tax Return	22,300,000
Federal Refund From Amending Second Prior Year Tax Return	3,500,000
Operating Loss Carryforwards	33,700,000
State Refund From Prior Year Tax Return	3,500,000
IRS Tax Payment Due To Examination	1,500,000
IRS Interest Payment Due To Examination	200,000
State Tax Assessment Under Review	800,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	37,550,000	42,745,000	42,627,000
Income Tax Reconciliation, State and Local Income Taxes	4,945,000	5,803,000	5,568,000
Income Tax Reconciliation, Tax Credits, Investment	(442,000)	(525,000)	(593,000)
Differences Regulatory Commission	1,647,000	1,647,000	(116,000)
Income Tax Reconciliation, Tax Credits, Other	(786,000)	(715,000)	(1,195,000)
Income Tax Reconciliation, Other Adjustments	468,000	507,000	380,000
Income tax expense	43,382,000	49,462,000	46,671,000
Effective Income Tax Rate Continuing Operations	40.40%	40.50%	38.30%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)	130,000	(28,592,000)	6,221,000
Current State and Local Tax Expense (Benefit)	(929,000)	1,441,000	2,300,000
Current Income Tax Expense (Benefit)	(799,000)	(27,151,000)	8,521,000
Deferred Federal Income Tax Expense (Benefit)	35,481,000	69,159,000	31,937,000
Deferred State and Local Income Tax Expense (Benefit)	8,700,000	7,454,000	6,213,000
Deferred Income Tax Expense (Benefit)	44,181,000	76,613,000	38,150,000
Income tax expense	43,382,000	49,462,000	46,671,000
Income taxes paid	1,756,000	22,600,000	10,000,000
Tax Provision Regulated And Nonutility [Abstract]
Regulated Current Tax Expense Benefit	(4,646,000)	(1,464,000)	871,000
Regulated Deferred And Local Tax Expense Benefit	50,152,000	47,741,000	40,829,000
Regulated Investments And Credits Tax Expense Benefit	(422,000)	(525,000)	(593,000)
Total Tax Provision Related To Regulated Utility	45,084,000	45,752,000	41,107,000
Non Utility Current Tax Expense Benefit	3,846,000	(25,687,000)	7,650,000
Non Utility Deferred And Local Tax Expense Benefit	(5,548,000)	29,397,000	(2,086,000)
Total Tax Provision Related To Non Utility	(1,702,000)	3,710,000	5,564,000
Income tax expense	$ 43,382,000	$ 49,462,000	$ 46,671,000