Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 117,410	$ 134,254
Regulatory balancing account revenue	8,701	299
Total operating revenue	$ 126,111	134,553
Maximum collection period in which deferred net WRAM and MCBA revenues and associated costs will be recognized	24 months
Total non-regulated revenue	$ 4,901	4,419
Lessor operating lease renewal term	15 years
2019	$ 3,032			$ 1,771
2020	2,482			1,709
2021	1,809			1,485
2022	1,013			1,355
2023	534			1,261
Thereafter	828			10,538
Cash and cash equivalents	60,234			47,176
Restricted cash (included in taxes, prepaid expenses and other assets)	656			539
Total cash, cash equivalents, and restricted cash shown in the statements of cash flows	60,890	35,333		$ 47,715	$ 95,352
Other assets	14,128
Operating lease liability	$ 14,112
ASU 2016-02
Summary of Significant Accounting Policies [Line Items]
Operating lease liability			$ 13,800
Minimum
Summary of Significant Accounting Policies [Line Items]
Lessor operating lease term	5 years
Maximum
Summary of Significant Accounting Policies [Line Items]
Lessor operating lease term	10 years
Regulated water and wastewater services
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	$ 117,410	134,254
Total operating revenue	126,111	134,553
Operating and maintenance revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	3,046	3,165
Other non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	1,296	743
Non-regulated revenue
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	4,342	3,908
Lease revenue	559	511
Total non-regulated revenue	4,901	4,419
Residential
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	84,259	91,319
Business
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	25,481	27,057
Industrial
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	7,264	7,579
Public authorities
Summary of Significant Accounting Policies [Line Items]
Revenue from contracts with customers	4,471	5,444
Other (a)
Summary of Significant Accounting Policies [Line Items]
Other	$ (4,065)	$ 2,855