Long-Term Debt (Details) (USD $)	12 Months Ended								12 Months Ended		12 Months Ended			0 Months Ended				0 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 GSWC	Dec. 31, 2011 GSWC	Dec. 31, 2010 GSWC	Dec. 31, 2012 Private placement notes	Dec. 31, 2011 Private placement notes	Dec. 31, 2012 Private placement notes GSWC	Dec. 31, 2011 Private placement notes GSWC	Dec. 31, 2012 Private placement notes GSWC Minimum	Dec. 31, 2012 Private placement notes GSWC Maximum	Dec. 31, 2011 7.55% notes due 2025	Oct. 02, 2012 7.55% notes due 2025 GSWC	Dec. 31, 2011 7.55% notes due 2025 GSWC	Dec. 31, 2012 6.00% notes due 2041	Dec. 31, 2011 6.00% notes due 2041	May 31, 2011 6.00% notes due 2041 GSWC	Dec. 31, 2012 6.00% notes due 2041 GSWC	Dec. 31, 2011 6.00% notes due 2041 GSWC	May 31, 2011 7.65% notes due 2025	Dec. 31, 2012 Senior Notes issued to Co-Bank	Dec. 31, 2012 Senior Notes issued to Co-Bank GSWC	Dec. 31, 2012 Debt issues other than 9.56% Notes and Senior Notes issued to Co-Bank GSWC Maximum	Dec. 31, 2012 American Recovery and Reinvestment Act Obligation due 2033	Dec. 31, 2011 American Recovery and Reinvestment Act Obligation due 2033	Dec. 31, 2012 American Recovery and Reinvestment Act Obligation due 2033 GSWC	Dec. 31, 2011 American Recovery and Reinvestment Act Obligation due 2033 GSWC
Long-term debt
Face amount of debt									$ 28,000,000	$ 28,000,000
Interest coverage ratio									300.00%		200.00%
Interest rate (as a percent)							9.56%	9.56%	9.56%	9.56%			7.55%	7.55%	7.55%	6.00%	6.00%		6.00%	6.00%							2.50%
Redemption price of debt instrument, as a percentage of principal amount														101.133%
Principal amount of notes redeemed														8,300,000
Amount of debt	335,791,000	340,686,000		335,791,000	340,686,000		28,000,000	28,000,000	28,000,000	28,000,000			8,000,000		8,000,000	62,000,000	62,000,000		62,000,000	62,000,000					4,259,000	851,000	4,259,000	851,000
Redemption premium on long-term debt																		421,000
Redemption premium (as a percent)																								0.765%
Additional spread on premium if notes redeemed prior to 2021 (as a percent)									0.55%
Make-whole redemption premium, base rate							applicable Treasury Yield															Cost of funds, difference between date of purchase and date of redemption
Redemption premium after 2021 (as a percent)												3.00%
Additional spread on premium (as a percent)																							0.50%
Retirement or repayments of long-term debt	(8,303,000)	(22,380,000)	(701,000)	(8,303,000)	(22,380,000)	(371,000)															(22,000,000)
Amount available for issuance of debt securities				$ 100,000,000