Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jun. 08, 2017	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
Operating revenues		$ 104,180	$ 124,418	$ 113,195	$ 98,810	$ 106,800	$ 123,806	$ 111,954	$ 93,527	$ 440,603	$ 436,087	$ 458,641
Operating income (loss)		21,955	38,567	41,816	24,738	22,092	39,617	31,774	21,233	127,076	114,716	118,489
Net Income		$ 12,868	$ 21,006	$ 22,792	$ 12,701	$ 11,212	$ 21,639	$ 16,742	$ 10,150	$ 69,367	$ 59,743	$ 60,484
Basic earnings (in dollars per share)		$ 0.35	$ 0.57	$ 0.62	$ 0.35	$ 0.30	$ 0.59	$ 0.46	$ 0.28	$ 1.88	$ 1.63	$ 1.61
Diluted earnings (in dollars per share)		$ 0.35	$ 0.57	$ 0.62	$ 0.34	$ 0.30	$ 0.59	$ 0.45	$ 0.28	$ 1.88	$ 1.62	$ 1.60
Gain on sale of properties	$ 8,300
GSWC
Operating revenues		$ 75,136	$ 99,913	$ 88,346	$ 76,906	$ 74,295	$ 98,763	$ 88,759	$ 76,885	$ 340,301	$ 338,702	$ 364,550
Operating income (loss)		15,824	33,068	35,079	22,077	14,477	34,142	27,557	19,643	106,048	95,819	100,409
Net Income		7,309	$ 17,336	18,363	$ 10,749	6,432	$ 17,883	$ 13,670	$ 8,984	$ 53,757	$ 46,969	$ 47,591
Gain on sale of properties				8,300
Cash received for reimbursement of legal and other defense costs		$ (1,800)
Restatement Adjustment | GSWC
Operating revenues						5,200
Restatement Adjustment | American States Utility Services [Member]
Operating revenues				$ 1,000		$ 1,700