Regulatory Matters (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Sep. 30, 2017	Oct. 31, 2005	Jul. 31, 2005	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Regulatory matters:
Litigation Settlement, Amount Awarded to Other Party	$ 2,600
Regulatory Asset Not Accruing Carrying Costs				$ 59,100
Pension Costs and Other Postretirement Benefit Costs
Regulatory matters:
Regulatory Asset Not Accruing Carrying Costs				34,700
General Rate Case Memorandum Accounts [Member]
Regulatory matters:
Total				9,500
GSWC
Regulatory matters:
Total				2,042		$ 146,281
GSWC | Various refunds to customers
Regulatory matters:
Total				$ (4,552)		(5,866)
GSWC | WRAM, net of MCBA
Regulatory matters:
Recovery Period for under Collection, Balances Greater than 15 Percent of Adopted Annual Revenues				18 months
Total				$ 29,556		47,340
GSWC | Costs deferred for future recovery on Aerojet case
Regulatory matters:
Total				10,656		11,820
Regulatory Asset Recovery Periods			20 years
GSWC | Pension Costs and Other Postretirement Benefit Costs
Regulatory matters:
Total				33,019		28,118
GSWC | Gain (Loss) on Derivative Instruments [Member]
Regulatory matters:
Total				2,941		4,901
GSWC | Flow-through taxes, net
Regulatory matters:
Total				(17,716)		20,134
GSWC | Deferred Income Tax Charge [Member]
Regulatory matters:
Total				(83,231)		0
GSWC | Low income rate assistance balancing accounts
Regulatory matters:
Total				5,972		8,272
GSWC | Other regulatory assets, net
Regulatory matters:
Total				14,875		17,633
GSWC | General Rate Case Memorandum Accounts [Member]
Regulatory matters:
Total				$ 10,522		$ 13,929
Minimum [Member] | GSWC
Regulatory matters:
Recovery Period for under Collection, Balances Greater than 15 Percent of Adopted Annual Revenues				19 months
Minimum [Member] | GSWC | WRAM, net of MCBA
Regulatory matters:
Recovery Period for under Collection, Balances Greater than 15 Percent of Adopted Annual Revenues				12 months
Minimum [Member] | GSWC | General Rate Case Memorandum Accounts [Member]
Regulatory matters:
Regulatory Asset Recovery Periods				12 months
Maximum | GSWC | WRAM, net of MCBA
Regulatory matters:
Recovery Period for under Collection, Balances Greater than 15 Percent of Adopted Annual Revenues				24 months
Alternative Revenue Program				24 months	24 months
Maximum | GSWC | Costs deferred for future recovery on Aerojet case
Regulatory matters:
Regulatory Asset Recovery Periods		20 years
Maximum | GSWC | General Rate Case Memorandum Accounts [Member]
Regulatory matters:
Regulatory Asset Recovery Periods				36 months