Interim Financial Statements - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Jun. 02, 2016	Sep. 30, 2013	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2017
Accounting Policies [Line Items]
Document Period End Date			Dec. 31, 2019
Number of options outstanding, to acquire shares of GWRI common stock (in shares)			633,000	498,000		740,000
Total proceeds from transfer of project agreement		$ 4,100
Other income (expense)			$ 1,309	$ 592
Deferred tax liabilities, gross			13,754	14,041
Net income			2,224	$ 3,103
Private Letter Ruling
Accounting Policies [Line Items]
Gain on condemnation of Valencia	$ 19,400
Valencia Water Company
Accounting Policies [Line Items]
Deferred tax liabilities, gross			$ 100
Employee Stock Option | 2016 stock option grant
Accounting Policies [Line Items]
Number of options outstanding, to acquire shares of GWRI common stock (in shares)					0
Employee Stock Option | 2017 stock option grant
Accounting Policies [Line Items]
Number of options outstanding, to acquire shares of GWRI common stock (in shares)			386,896