Depreciation and Amortization, by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 15,090	$ 18,807	$ 29,468	$ 35,481
Electricity Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	5,774	7,771	11,085	15,033
Gas Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	4,503	4,968	8,747	9,889
Water Operating Segment [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	3,887	4,636	7,846	9,018
Corporate Unallocated [Member]
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 926	$ 1,432	$ 1,790	$ 1,541