Segment Information Information By Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 550,776	$ 486,747	$ 503,082	$ 477,592	$ 495,713	$ 506,859	$ 513,024	$ 497,590	$ 2,018,197	$ 2,013,186	$ 1,883,533
Gross Profit	$ 174,751	$ 165,318	$ 177,860	$ 157,225	$ 156,663	$ 170,749	$ 169,705	$ 163,203	675,154	660,320	556,685
Operating Income (Loss)									151,426	96,211	52,846
Income before income taxes									134,575	84,627	37,102
Depreciation and amortization									63,215	68,318	75,993
Total other income (expense)									(16,851)	(11,584)	(15,744)
Electricity Operating Segment [Member]
Segment Reporting Information [Line Items]
Revenues									1,022,939	938,374	820,306
Gross Profit									318,953	282,677	225,446
Operating Income (Loss)									93,566	68,287	31,104
Depreciation and amortization									24,703	28,468	35,896
Gas Operating Segment [Member]
Segment Reporting Information [Line Items]
Revenues									533,624	569,476	543,805
Gross Profit									191,303	205,063	185,559
Operating Income (Loss)									74,206	66,813	67,471
Depreciation and amortization									18,800	20,714	20,288
Water Operating Segment [Member]
Segment Reporting Information [Line Items]
Revenues									461,634	505,336	519,422
Gross Profit									164,898	172,580	145,680
Operating Income (Loss)									44,494	37,266	19,864
Depreciation and amortization									16,092	18,675	19,459
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)									(60,840)	(76,155)	(65,593)
Depreciation and amortization									$ 3,620	$ 461	$ 350