Regulatory assets and liabilities (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Regulatory assets and liabilities
Regulatory assets:		$ 307,320	$ 263,258
Regulatory liabilities:		241,852	243,118
Net regulatory position		65,468	20,140
Regulatory assets not earning a rate of return		255,400	224,700
Plant removal and decommissioning costs [Member}
Regulatory assets and liabilities
Regulatory liabilities:	[1]	176,972	182,981
Taxes refundable to customers [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	11,010	17,060
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	9,099	4,764
Natural gas costs refundable through rate adjustments [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[2]	25,580	20,884
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1],[2]	$ 19,191	17,429
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Estimated recovery period	[3]	(e)
Regulatory assets:	[4]	$ 176,025	185,832
Taxes recoverable from customers [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Over plant lives
Regulatory assets:	[4]	$ 28,278	27,682
Manufactured gas plant sites remediation [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	0
Regulatory assets:	[4]	$ 18,259	18,617
Asset Retirement Obligation Costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	0
Regulatory assets:	[4]	$ 42,580	8,000
Natural gas costs recoverable through rate adjustments [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 1 year
Regulatory assets:	[6]	$ 2,242	547
Long-term debt refinancing costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 21 years
Regulatory assets:	[4]	$ 6,248	7,031
Costs related to identifying generation development [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 10 years
Regulatory assets:	[4]	$ 3,407	3,808
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Largely within 1- 4 years
Regulatory assets:	[4],[6]	$ 30,281	$ 11,741

[1] Included in deferred credits and other liabilities - other 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.