Regulatory assets and liabilities (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Regulatory assets and liabilities
Regulatory assets:		$ 300,963	$ 307,320
Regulatory liabilities:		519,298	241,852
Net regulatory position		(218,335)	65,468
Regulatory assets not earning a rate of return		269,100	255,400
Plant removal and decommissioning costs [Member}
Regulatory assets and liabilities
Regulatory liabilities:	[1]	176,190	176,972
Taxes recoverable from/refundable to customers [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	279,668	11,010
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1]	11,056	9,099
Natural gas costs refundable through rate adjustments [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[2]	28,514	25,580
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Regulatory liabilities:	[1],[2]	$ 23,870	19,191
Pension and postretirement benefits [Member]
Regulatory assets and liabilities
Estimated recovery period	[3]	(e)
Regulatory assets:	[4]	$ 163,896	176,025
Taxes recoverable from/refundable to customers [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Over plant lives
Regulatory assets:	[4]	$ 12,073	28,278
Manufactured gas plant sites remediation [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	-
Regulatory assets:	[4]	$ 18,213	18,259
Asset retirement obligation costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Over plant lives
Regulatory assets:	[4]	$ 56,078	42,580
Natural gas costs recoverable through rate adjustments [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 1 year
Regulatory assets:	[6]	$ 14,465	2,242
Long-term debt refinancing costs [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 20 years
Regulatory assets:	[4]	$ 5,563	6,248
Costs related to identifying generation development [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 9 years
Regulatory assets:	[4]	$ 2,960	3,407
Other regulatory assets/liabilities [Member]
Regulatory assets and liabilities
Estimated recovery period	[5]	Up to 20 years
Regulatory assets:	[4],[6]	$ 27,715	$ 30,281
Other noncurrent assets [Member] | Stranded costs [Member]
Regulatory assets and liabilities
Deferred tax liabilities, regulatory assets		15,500
Other noncurrent liabilities [Member] | Stranded costs [Member]
Regulatory assets and liabilities
Deferred tax liabilities, regulatory assets		$ 270,000

[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.