Employee Benefit Plans: (Details) (USD $)	1 Months Ended	3 Months Ended
	Apr. 30, 2013	Mar. 31, 2013	Mar. 31, 2012
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred		$ 1,371,000	$ 3,437,000
GSWC
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred		1,371,000	3,437,000
Pension Benefits
Components of Net Periodic Benefits Cost:
Service cost		1,864,000	1,719,000
Interest cost		1,731,000	1,676,000
Expected return on plan assets		(1,893,000)	(1,636,000)
Amortization of prior service cost (benefit)		30,000	30,000
Amortization of actuarial loss		711,000	778,000
Net periodic pension cost under accounting standards		2,443,000	2,567,000
Regulatory adjustment - deferred		(510,000)	(566,000)
Total expense recognized, before allocation to overhead pool		1,933,000	2,001,000
Expected employer's contribution in 2013		6,600,000
Employer contributions	2,100,000	0
Pension Benefits | GSWC
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred		(4,900,000)
Post-Retirement Medical Benefits
Components of Net Periodic Benefits Cost:
Service cost		106,000	112,000
Interest cost		113,000	136,000
Expected return on plan assets		(95,000)	(90,000)
Amortization of transition		105,000	105,000
Amortization of prior service cost (benefit)		(50,000)	(50,000)
Net periodic pension cost under accounting standards		179,000	213,000
Total expense recognized, before allocation to overhead pool		179,000	213,000
Expected employer's contribution in 2013		150,000
Employer contributions		0
SERP
Components of Net Periodic Benefits Cost:
Service cost		201,000	183,000
Interest cost		129,000	122,000
Amortization of prior service cost (benefit)		40,000	40,000
Amortization of actuarial loss		85,000	77,000
Net periodic pension cost under accounting standards		455,000	422,000
Total expense recognized, before allocation to overhead pool		$ 455,000	$ 422,000