Employee Benefit Plans: (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost	$ 1,670,000	$ 1,406,000	$ 5,007,000	$ 4,218,000
Interest cost	1,663,000	1,631,000	4,992,000	4,893,000
Expected return on plan assets	(1,634,000)	(1,587,000)	(4,905,000)	(4,761,000)
Amortization of prior service cost (benefit)	31,000	30,000	90,000	89,000
Amortization of actuarial loss	759,000	310,000	2,277,000	931,000
Net periodic pension cost under accounting standards	2,489,000	1,790,000	7,461,000	5,370,000
Regulatory adjustment - deferred	(596,000)	(127,000)	(1,794,000)	(380,000)
Total expense recognized, before allocation to overhead pool	1,893,000	1,663,000	5,667,000	4,990,000
Employer's contribution and fair value of plan assets
Contributed to the pension plan	4,300,000		6,100,000
Pension Benefits | GSWC
Regulatory Adjustment:
Under-collection in the two-way pension balancing account	3,700,000		3,700,000
Other Postretirement Benefits
Components of Net Periodic Benefits Cost:
Service cost	112,000	107,000	336,000	321,000
Interest cost	136,000	153,000	408,000	459,000
Expected return on plan assets	(90,000)	(74,000)	(270,000)	(221,000)
Amortization of transition	105,000	105,000	315,000	315,000
Amortization of prior service cost (benefit)	(50,000)	(50,000)	(150,000)	(150,000)
Net periodic pension cost under accounting standards	213,000	241,000	639,000	724,000
Total expense recognized, before allocation to overhead pool	213,000	241,000	639,000	724,000
Employer's contribution and fair value of plan assets
Expected contribution during the fourth quarter			500,000
SERP
Components of Net Periodic Benefits Cost:
Service cost	183,000	150,000	549,000	450,000
Interest cost	122,000	116,000	366,000	348,000
Amortization of prior service cost (benefit)	40,000	40,000	120,000	121,000
Amortization of actuarial loss	77,000	34,000	231,000	101,000
Net periodic pension cost under accounting standards	422,000	340,000	1,266,000	1,020,000
Total expense recognized, before allocation to overhead pool	$ 422,000	$ 340,000	$ 1,266,000	$ 1,020,000