Regulatory assets and liabilities (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Regulatory assets and liabilities
Regulatory assets:	$ 351,194,000		$ 359,009,000
Regulatory liabilities:	450,482,000		460,229,000
Net regulatory position	(99,288,000)		(101,220,000)
Regulatory assets not earning a rate of return	215,600,000		216,400,000
Plant removal and decommissioning costs (c)
Regulatory assets and liabilities
Regulatory liabilities:	296,037,000	[1]	289,972,000	[1]
Deferred income taxes
Regulatory assets and liabilities
Regulatory liabilities:	82,077,000	[2]	84,963,000	[2]
Natural gas costs refundable through rate adjustments (d)
Regulatory assets and liabilities
Regulatory liabilities:	35,328,000	[3]	45,064,000	[3]
Taxes refundable to customers (c)
Regulatory assets and liabilities
Regulatory liabilities:	24,212,000	[1]	31,837,000	[1]
Other (c) (d)
Regulatory assets and liabilities
Regulatory liabilities:	12,828,000	[1],[3]	8,393,000	[1],[3]
Pension and postretirement benefits (a)
Regulatory assets and liabilities
Estimated recovery period	(e)	[4]
Regulatory assets:	166,477,000	[4]	171,492,000	[4]
Deferred income taxes
Regulatory assets and liabilities
Estimated recovery period	**	[2]
Regulatory assets:	121,781,000	[2]	119,189,000	[2]
Manufactured gas plant sites remediation (a)
Regulatory assets and liabilities
Estimated recovery period	Up to 5 years	[5]
Regulatory assets:	15,828,000	[6]	8,150,000	[6]
Plant costs (a)
Regulatory assets and liabilities
Estimated recovery period	Over plant lives	[5]
Regulatory assets:	10,348,000	[6]	10,256,000	[6]
Long-term debt refinancing costs (a)
Regulatory assets and liabilities
Estimated recovery period	Up to 25 years	[5]
Regulatory assets:	9,144,000	[6]	10,112,000	[6]
Taxes recoverable from customers (a)
Regulatory assets and liabilities
Estimated recovery period	0	[5]
Regulatory assets:	9,078,000	[6]	12,433,000	[6]
Costs related to identifying generation development (a)
Regulatory assets and liabilities
Estimated recovery period	Up to 14 years	[5]
Regulatory assets:	5,773,000	[6]	9,817,000	[6]
Other (a) (b)
Regulatory assets and liabilities
Estimated recovery period	Largely within 1 year	[5]
Regulatory assets:	$ 12,765,000	[6],[7]	$ 17,560,000	[6],[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] Recovered as expense is incurred.

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

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

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