CONSOLIDATED STATEMENTS OF CHANGES IN COMMON SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	GSWC	Common Stock [Member]	Common Stock [Member] GSWC	Earnings Reinvested in the Business	Earnings Reinvested in the Business GSWC
Balances at Dec. 31, 2014	$ 506,801	$ 435,190	$ 253,199	$ 235,607	$ 253,602	$ 199,583
Balances (in shares) at Dec. 31, 2014			38,287,000	146
Balances (in shares) at Dec. 31, 2015			36,502,000	146
Add:
Net Income	60,484	47,591			60,484	47,591
Issuance of Common Shares	$ 1,198		$ 1,198
Issuance of Common Shares (in shares)	53,612		120,000
Exercise of stock options (in shares)	66,458
Tax benefit from employee stock-based awards	$ 877	872	$ 877	$ 872
Compensation on stock-based awards	2,168	2,077	2,168	2,077
Dividend equivalent rights on stock-based awards not paid in cash	270	239	$ 270	239
Stock Repurchased During Period, Shares			1,905,000
Stock Repurchased During Period, Value	72,893		$ 12,690		60,203
Deduct:
Dividends on Common Shares	32,690	62,000			32,690	62,000
Dividend equivalent rights on stock-based awards	270	239			270	239
Balances at Dec. 31, 2015	465,945	423,730	$ 245,022	$ 238,795	220,923	184,935
Balances (in shares) at Dec. 31, 2016			36,571,000	146
Add:
Net Income	$ 59,743	46,969			59,743	46,969
Issuance of Common Shares		501		$ 501
Issuance of Common Shares (in shares)	56,900
Exercise of stock options (in shares)	12,546
Exercise of stock options and other issuance of Common Shares	$ 235		$ 235
Exercise of stock options and other issuance of Common Shares (in shares)			69,000
Tax benefit from employee stock-based awards	581		$ 581
Compensation on stock-based awards	1,201	1,020	1,201	1,020
Dividend equivalent rights on stock-based awards not paid in cash	193	166	193	166
Deduct:
Dividends on Common Shares	33,408	25,450			33,408	25,450
Dividend equivalent rights on stock-based awards	193	166			193	166
Balances at Dec. 31, 2016	494,297	446,770	$ 247,232	$ 240,482	247,065	206,288
Balances (in shares) at Dec. 31, 2017			36,681,000	146
Add:
Net Income	$ 69,367	53,757				53,757
Issuance of Common Shares (in shares)	56,498
Exercise of stock options (in shares)	52,936
Exercise of stock options and other issuance of Common Shares	$ 909		$ 909
Exercise of stock options and other issuance of Common Shares (in shares)			110,000
Compensation on stock-based awards	1,789	1,527	$ 1,789	$ 1,527
Dividend equivalent rights on stock-based awards not paid in cash	194	172	194	172
Deduct:
Dividends on Common Shares	36,417	27,680			36,417	27,680
Dividend equivalent rights on stock-based awards	194	172			194	172
Balances at Dec. 31, 2017	$ 529,945	$ 474,374	$ 250,124	$ 242,181	$ 279,821	$ 232,193