Regulatory Assets and Liabilities (Details) (USD $)	Dec. 31, 2011		Dec. 31, 2010
Regulatory Assets and Liabilities
Regulatory Assets	$ 359,009,000		$ 316,241,000
Regulatory Liabilities	460,229,000		413,097,000
Net Regulatory position	(101,220,000)		(96,856,000)
Regulatory assets not earning a rate of return	216,400,000
Plant removal and decommissioning costs [Member]
Regulatory Assets and Liabilities
Regulatory Liabilities	289,972,000	[1]	276,652,000	[1]
Deferred income taxes liability [Member]
Regulatory Assets and Liabilities
Regulatory Liabilities	84,963,000	[2]	64,017,000	[2]
Liabilities Refundable Gas Costs [Member]
Regulatory Assets and Liabilities
Regulatory Liabilities	45,064,000	[3]	36,996,000	[3]
Taxes refundable to customers [Member]
Regulatory Assets and Liabilities
Regulatory Liabilities	31,837,000	[1]	19,352,000	[1]
Other regulatory liabilities [Member]
Regulatory Assets and Liabilities
Regulatory Liabilities	8,393,000	[1],[3]	16,080,000	[1],[3]
Pension and postretirement benefits [Member]
Regulatory Assets and Liabilities
Regulatory Assets	171,492,000	[4],[5]	103,818,000	[4],[5]
Deferred income taxes asset [Member]
Regulatory Assets and Liabilities
Regulatory Assets	119,189,000	[2]	114,427,000	[2]
Deferred Income Tax Charges [Member]
Regulatory Assets and Liabilities
Regulatory Assets	12,433,000	[4]	11,961,000	[4]
Plant costs [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Over plant lives	[6]
Regulatory Assets	10,256,000	[4]	9,964,000	[4]
Long-term debt refinancing costs [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 27 years	[6]
Regulatory Assets	10,112,000	[4]	11,101,000	[4]
Costs related to identifying generation development [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 15 years	[6]
Regulatory Assets	9,817,000	[4]	13,777,000	[4]
Natural gas supply derivatives [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 1 year	[6]
Regulatory Assets	437,000	[7]	9,359,000	[7]
Natural gas cost recoverable through rate adjustments [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Up to 28 months	[6]
Regulatory Assets	2,622,000	[7]	6,609,000	[7]
Other regulatory assets [Member]
Regulatory Assets and Liabilities
Estimated Recovery Period	Largely within 1 year	[6]
Regulatory Assets	$ 22,651,000	[4],[7]	$ 35,225,000	[4],[7]

[1] Included in other liabilities on the Consolidated Balance Sheets.

[2] Represents deferred income taxes related to regulatory assets and liabilities. The deferred income tax assets are not earning a rate of return.

[3] Included in other accrued liabilities on the Consolidated Balance Sheets.

[4] Included in deferred charges and other assets on the Consolidated Balance Sheets.

[5] Recovered as expense is incurred.

[6] Estimated recovery period for regulatory assets currently being recovered in rates charged to customers.

[7] Included in prepayments and other current assets on the Consolidated Balance Sheets.