Employee Benefit Plans (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amounts recognized on the balance sheets:
Non-current liabilities	$ (71,618)	$ (68,122)
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred	11,709	18,748	20,622
GSWC
Amounts recognized on the balance sheets:
Non-current liabilities	(71,618)	(68,122)
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred	11,709	18,748	20,622
Pension Benefits
Change in Projected Benefit Obligation:
Projected benefit obligation at beginning of year	146,127	118,766
Service cost	6,675	5,624	4,795
Interest cost	6,657	6,524	6,104
Actuarial loss (gain)	7,759	18,719
Benefits/expenses paid	(3,988)	(3,506)
Projected benefit obligation at end of year	163,230	146,127	118,766
Changes in Plan Assets:
Fair value of plan assets at beginning of year	92,910	90,179
Actual return on plan assets	12,599	(326)
Employer contributions	6,117	6,563
Benefits/expenses paid	(3,988)	(3,506)
Fair value of plan assets at end of year	107,638	92,910	90,179
Funded Status:
Net amount recognized as accrued pension cost	(55,592)	(53,217)
Amounts recognized on the balance sheets:
Non-current liabilities	(55,592)	(53,217)
Net amount recognized	(55,592)	(53,217)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)	404	522
Net loss	48,648	49,983
Regulatory assets	49,052	50,505	26,471
Unfunded accrued pension cost	6,540	2,712
Net liability recognized	55,592	53,217
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	50,505	26,471
Net loss (gain)	1,700	25,394
Amortization of prior service (cost) credit	(118)	(118)
Amortization of net loss	(3,035)	(1,242)
Total change in regulatory asset	(1,453)	24,034
Regulatory asset at end of year	49,052	50,505	26,471
Net periodic pension costs	9,945	7,159	6,934
Change in regulatory asset	(1,453)	24,034
Total recognized in net periodic pension cost and regulatory asset	8,492	31,193
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service cost	(118)	(118)
Net loss	(2,846)	(3,114)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	163,230	146,127
Accumulated benefit obligation	138,230	122,399
Fair value of plan assets	107,638	92,910
Weighted-average assumptions used to determine benefit obligations at year end:
Discount rate (as a percent)	4.30%	4.65%
Rate of compensation increase (as a percent)	4.00%	4.00%
Components of Net Periodic Benefits Cost:
Service cost	6,675	5,624	4,795
Interest cost	6,657	6,524	6,104
Expected return on plan assets	(6,540)	(6,349)	(5,253)
Amortization of prior service cost (benefit)	118	118	118
Amortization of actuarial loss	3,035	1,242	1,170
Net periodic pension cost under accounting standards	9,945	7,159	6,934
Regulatory adjustment - deferred	(2,305)	(209)	(1,823)
Total expense recognized, before allocation to overhead pool	7,640	6,950	5,111
Weighted-average assumptions used to determine net periodic cost:
Discount rate (as a percent)	4.65%	5.55%	6.05%
Expected long-term return on plan assets (as a percent)	7.00%	7.00%	7.00%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%
Pension Benefits | GSWC
Components of Net Periodic Benefits Cost:
Regulatory adjustment - deferred	2,300	209	1,800
Post-Retirement Medical Benefits
Change in Projected Benefit Obligation:
Projected benefit obligation at beginning of year	12,448	12,057
Service cost	419	391	390
Interest cost	534	549	626
Actuarial loss (gain)	(657)	(236)
Benefits/expenses paid	(448)	(313)
Projected benefit obligation at end of year	12,296	12,448	12,057
Changes in Plan Assets:
Fair value of plan assets at beginning of year	7,916	6,959
Actual return on plan assets	736	319
Employer contributions	155	951
Benefits/expenses paid	(448)	(313)
Fair value of plan assets at end of year	8,359	7,916	6,959
Funded Status:
Net amount recognized as accrued pension cost	(3,937)	(4,532)
Amounts recognized on the balance sheets:
Non-current liabilities	(3,937)	(4,532)
Net amount recognized	(3,937)	(4,532)
Amounts recognized in regulatory assets consist of:
Initial net obligation	837	1,256
Prior service cost (credit)	(633)	(833)
Net loss	(889)	147
Regulatory assets	(685)	570	1,045
Unfunded accrued pension cost	4,622	3,962
Net liability recognized	3,937	4,532
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	570	1,045
Net loss (gain)	(1,036)	(256)
Amortization of initial net obligation	(419)	(419)
Amortization of prior service (cost) credit	200	200
Total change in regulatory asset	(1,255)	(475)
Regulatory asset at end of year	(685)	570	1,045
Net periodic pension costs	815	861	980
Change in regulatory asset	(1,255)	(475)
Total recognized in net periodic pension cost and regulatory asset	(440)	386
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Initial net obligation	(419)	(419)
Prior service cost	200	200
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	12,296	12,448
Fair value of plan assets	8,359	7,916
Weighted-average assumptions used to determine benefit obligations at year end:
Discount rate (as a percent)	3.75%	4.45%
Components of Net Periodic Benefits Cost:
Service cost	419	391	390
Interest cost	534	549	626
Expected return on plan assets	(357)	(298)	(255)
Amortization of transition	419	419	419
Amortization of prior service cost (benefit)	(200)	(200)	(200)
Net periodic pension cost under accounting standards	815	861	980
Total expense recognized, before allocation to overhead pool	815	861	980
Weighted-average assumptions used to determine net periodic cost:
Discount rate (as a percent)	4.45%	5.20%	5.85%
Union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets (as a percent)	7.00%	7.00%	7.00%
Non-union plan
Weighted-average assumptions used to determine net periodic cost:
Expected long-term return on plan assets (as a percent)	4.20%	4.20%	4.20%
SERP
Change in Projected Benefit Obligation:
Projected benefit obligation at beginning of year	10,604	8,468
Service cost	731	600	433
Interest cost	488	464	354
Actuarial loss (gain)	803	1,292
Projected benefit obligation at end of year	12,406	10,604	8,468
Funded Status:
Net amount recognized as accrued pension cost	(12,406)	(10,604)
Amounts recognized on the balance sheets:
Current liabilities	(317)	(231)
Non-current liabilities	(12,089)	(10,373)
Net amount recognized	(12,406)	(10,604)
Amounts recognized in regulatory assets consist of:
Prior service cost (credit)	475	636
Net loss	3,709	3,212
Regulatory assets	4,184	3,848	2,852
Unfunded accrued pension cost	8,222	6,756
Net liability recognized	12,406	10,604
Changes in plan assets and benefit obligations recognized in regulatory assets:
Regulatory asset at beginning of year	3,848	2,852
Net loss (gain)	805	1,291
Amortization of prior service (cost) credit	(161)	(161)
Amortization of net loss	(308)	(134)
Total change in regulatory asset	336	996
Regulatory asset at end of year	4,184	3,848	2,852
Net periodic pension costs	1,687	1,359	948
Change in regulatory asset	336	996
Total recognized in net periodic pension cost and regulatory asset	2,023	2,355
Estimated amounts that will be amortized from regulatory asset over the next fiscal year:
Prior service cost	(161)	(161)
Net loss	(339)	(307)
Additional year-end information for plans with an accumulated benefit obligation in excess of plan assets:
Projected benefit obligation	12,406	10,604
Accumulated benefit obligation	9,601	7,639
Weighted-average assumptions used to determine benefit obligations at year end:
Discount rate (as a percent)	4.20%	4.65%
Rate of compensation increase (as a percent)	4.00%	4.00%
Components of Net Periodic Benefits Cost:
Service cost	731	600	433
Interest cost	488	464	354
Amortization of prior service cost (benefit)	161	161	161
Amortization of actuarial loss	307	134
Net periodic pension cost under accounting standards	$ 1,687	$ 1,359	$ 948
Weighted-average assumptions used to determine net periodic cost:
Discount rate (as a percent)	4.65%	5.55%	6.05%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%