Depreciation and Amortization, by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 16,532	$ 16,082	$ 46,000	$ 51,563
Electricity Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	6,687	6,682	17,772	21,715
Gas Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	5,084	5,595	13,831	15,484
Water Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	3,860	5,022	11,706	14,040
Corporate Unallocated [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 901	$ (1,217)	$ 2,691	$ 324