Income Tax (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred Tax Assets And Liabilities [Abstract]
Deferred Tax Liabilities, Property, Plant and Equipment	$ 322,527,000	$ 292,235,000
Deferred Tax Liabilities Regulatory Taxes Paid	0	2,106,000
Deferred Tax Liabilities, Regulatory Assets	60,253,000	65,755,000
Deferred Tax Liabilities, Regulatory Assets and Liabilities	51,424,000	35,638,000
Deferred Tax Liabilities Non Regulated Liabilities	43,824,000	43,373,000
Deferred Tax Liabilities, Net	478,028,000	439,107,000
Deferred Tax Assets, Regulatory Assets and Liabilities	(7,724,000)	4,727,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits	6,024,000	5,119,000
Deferred Tax Assets Non Regulated Assets	(1,235,000)	1,161,000
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax	1,986,000	1,626,000
Deferred Tax Assets, Tax Credit Carryforwards, Other	32,997,000	14,255,000
Deferred Tax Assets, Net	32,048,000	26,888,000
Deferred Tax Assets (Liabilities), Net	445,980,000	412,219,000
Accumulated Deferred Investment Tax Credit	624,000	990,000
Deferred Tax Liabilities	446,604,000	413,209,000
Income Tax Disclosure [Abstract]
One-Time Tax Adjustment, Net of Federal Benefit	2,700,000
Federal Operating Loss Carryfoward	83,400,000
State Operating Loss Carryforward	76,600,000
IRS Tax Payment Due To Examination	200,000
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Tax Reconciliation, Income Tax Expense (Benefit), at Federal Statutory Income Tax Rate	36,386,000	37,550,000	42,745,000
Income Tax Reconciliation, State and Local Income Taxes	4,773,000	4,945,000	5,803,000
Income Tax Reconciliation, Tax Credits, Investment	(350,000)	(442,000)	(525,000)
Differences Regulatory Commission	1,718,000	1,647,000	1,647,000
Income Tax Reconciliation, Tax Credits, Other	(800,000)	(786,000)	(715,000)
One-Time Tax Adjustment, Net of Federal Benefit	2,700,000
Income Tax Reconciliation, Other Adjustments	(323,000)	468,000	507,000
Income Tax Expense (Benefit)	44,104,000	43,382,000	49,462,000
Effective Income Tax Rate	42.40%	40.40%	40.50%
Tax Provision Current And Deferred [Abstract]
Current Federal Tax Expense (Benefit)	1,693,000	130,000	(28,592,000)
Current State and Local Tax Expense (Benefit)	99,000	(929,000)	1,441,000
Current Income Tax Expense (Benefit)	1,792,000	(799,000)	(27,151,000)
Deferred Federal Income Tax Expense (Benefit)	31,767,000	35,481,000	69,159,000
Deferred State and Local Income Tax Expense (Benefit)	10,545,000	8,700,000	7,454,000
Deferred Income Tax Expense (Benefit)	42,312,000	44,181,000	76,613,000
Income Tax Expense (Benefit)	44,104,000	43,382,000	49,462,000
Income Taxes Paid	2,979,000	1,756,000	22,600,000
Tax Provision Regulated And Nonutility [Abstract]
Regulated Current Tax Expense Benefit	1,909,000	(4,646,000)	(1,464,000)
Regulated Deferred And Local Tax Expense Benefit	39,864,000	50,152,000	47,741,000
Regulated Investments And Credits Tax Expense Benefit	(350,000)	(422,000)	(525,000)
Total Tax Provision Related To Regulated Utility	41,423,000	45,084,000	45,752,000
Non Utility Current Tax Expense Benefit	(117,000)	3,846,000	(25,687,000)
Non Utility Deferred And Local Tax Expense Benefit	2,798,000	(5,548,000)	29,397,000
Total Tax Provision Related To Non Utility	2,681,000	(1,702,000)	3,710,000
Income Tax Expense (Benefit)	$ 44,104,000	$ 43,382,000	$ 49,462,000