Long-Term Water Assets - Volume of Water Assets (Details)	Mar. 31, 2018 acre ft	Dec. 31, 2017 acre ft	Dec. 31, 2009 acre ft	Dec. 31, 2008 acre ft
Banked water and water for future delivery
Water contracts and purchased water	10,137,000	10,137,000
Total purchased water contracts	80,617,000	80,617,000
Total purchased and contracted water sources in acre feet	132,508,000	131,316,000
AVEK
Banked water and water for future delivery
AVEK water bank	13,033,000	13,033,000	6,393	8,393
Company water bank	33,111,000	31,497,000
Transferable water	5,747,000	6,169,000
Banked water and water for future delivery	51,891,000	50,699,000
Wheeler Ridge Maricopa Water Storage District
Banked water and water for future delivery
Water contracts and purchased water	15,547,000	15,547,000
Tejon-Castac Water District
Banked water and water for future delivery
Banked water and water for future delivery	49,184,000	49,184,000
Water contracts and purchased water	5,749,000	5,749,000
Transferable water
Banked water and water for future delivery
AVEK water bank	5,747
Transferable water | AVEK
Banked water and water for future delivery
AVEK water bank	349
Transferable water	523
Transferable water with AVEK multiple	1.5