Regulatory assets and liabilities (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Regulatory assets and liabilities
Regulatory assets:		$ 263,258	$ 269,586
Regulatory liabilities:		243,118	386,252
Net regulatory position		20,140	(116,666)
Regulatory assets not earning a rate of return		224,700	229,600
Plant removal and decommissioning costs [Member}
Regulatory assets and liabilities
Regulatory liabilities:	[1]	182,981	338,641
Taxes refundable to customers [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	17,060	17,772
Natural gas costs refundable through rate adjustments [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[2]	20,884	13,238
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1],[2]	$ 22,193	16,601
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Estimated recovery period	[3]	(e)
Regulatory assets:	[4]	$ 185,832	182,565
Taxes recoverable from customers [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Over plant lives
Regulatory assets:	[4]	$ 27,682	22,910
Manufactured gas plant sites remediation [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 2 years
Regulatory assets:	[4]	$ 18,617	17,548
Plant costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 1 year
Regulatory assets:	[4]	$ 8,000	4,551
Natural gas costs recoverable through rate adjustments [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 1 year
Regulatory assets:	[6]	$ 547	19,575
Long-term debt refinancing costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 22 years
Regulatory assets:	[4]	$ 7,031	7,864
Costs related to identifying generation development [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 11 years
Regulatory assets:	[4]	$ 3,808	4,165
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Largely within 1- 4 years
Regulatory assets:	[4],[6]	$ 11,741	$ 10,408

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

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

[3] Recovered as expense is incurred or cash contributions are made.

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

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

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