Long-Term Water Assets - Volume of Water Assets (Details) - acre ft	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2009	Dec. 31, 2008
Banked water and water for future delivery
Transferable water	6,082,000	6,169,000
Banked water and water for future delivery	101,933,000	99,883,000
Total water contracts	31,433,000	31,433,000
Total purchased and contracted water sources in acre feet	133,366,000	131,316,000
AVEK water bank
Banked water and water for future delivery
AVEK water bank	13,033,000	13,033,000	6,393	8,393
Company water bank
Banked water and water for future delivery
Company water bank	33,634,000	31,497,000
TCWD
Banked water and water for future delivery
TCWD - Banked water contracted with Company	49,184,000	49,184,000
Water contracts and purchased water	5,749,000	5,749,000
Dudley-Ridge, Nickel and Tulare
Banked water and water for future delivery
Water contracts and purchased water	10,137,000	10,137,000
WRMWSD
Banked water and water for future delivery
Water contracts and purchased water	15,547,000	15,547,000