Unaudited Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenue	$ 62,449	[1]	$ 82,374	$ 65,575	$ 51,149	$ 62,338	[1]	$ 73,914	$ 59,007	$ 43,696	$ 261,547	$ 238,955	$ 215,638
Operating Income (Loss)	14,110	[1]	20,877	13,585	6,725	16,520	[1]	18,486	13,784	5,601	55,297	54,391	38,075
Net income	5,924	[1]	10,084	5,201	1,109	6,602	[1]	8,215	5,451	610	22,318	20,878	24,406
Comprehensive income	5,871	[1]	10,125	5,260	1,098	6,727	[1]	7,988	5,479	599	22,354	20,793	13,578
Earnings per share
—Basic (in dollars per share)	$ 0.32	[1]	$ 0.54	$ 0.28	$ 0.06	$ 0.36	[1]	$ 0.44	$ 0.29	$ 0.03	$ 1.20	$ 1.12	$ 1.32
—Diluted (in dollars per share)	$ 0.31	[1]	$ 0.53	$ 0.28	$ 0.06	$ 0.35	[1]	$ 0.44	$ 0.29	$ 0.03	$ 1.18	$ 1.11	$ 1.30
Dividends per share	$ 0.18	[1]	$ 0.18	$ 0.18	$ 0.18	$ 0.17	[1]	$ 0.17	$ 0.17	$ 0.17
Balancing and memorandum accounts, revenue and regulatory assets	3,550
Balancing and Memorandum Accounts, Net Overcollection of Revenues	240
Balancing and Memorandum Accounts, Net Undercollection of Revenues	3,790
Memorandum Account, Recovery Revenue Recognized						$ 5,740
--- High (in dollars per share) [Member]
Earnings per share
Market Price, Stock	$ 26.62	[1]	$ 25.64	$ 24.34	$ 24.91	$ 24.93	[1]	$ 24.80	$ 24.24	$ 26.40
--- Low (in dollars per share) [Member]
Earnings per share
Market Price, Stock	$ 22.69	[1]	$ 22.95	$ 22.81	$ 22.96	$ 21.10	[1]	$ 21.16	$ 21.99	$ 22.48

[1] During the quarter ended December 31, 2012, the Company recorded revenues of $3,550 related to the recognition of certain San Jose Water Company balancing and memorandum accounts as the Company concluded that it was probable that these amounts would be collected. Of this amount, $240 represents a net over-collection of revenues generated during the quarter ended December 31, 2012 and $3,790 represents a net-under-collection of revenues which should have been recognized as revenue during the quarter ended September 30, 2012. See Note 1 of the Notes to the Consolidated Financial Statements for further discussion on balancing and memorandum accounts. During the quarter ended December 31, 2011, the Company recorded revenues of $5,740 related to the recognition of San Jose Water Company's MCRAM which was authorized for recovery by the CPUC on December 27, 2011.