Investments in Water Assets - Volume of Water Assets (Details)	Sep. 30, 2017 acre ft	Dec. 31, 2016 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 intangible water assets	75,017,000	64,823,000
Total purchased and contracted water sources in acre feet	130,397,000	106,567,000
AVEK
Banked water and water for future delivery
AVEK water bank	13,033,000	13,033,000	6,393	8,393
Company water bank	26,338,000	17,287,000
AVEK water for future delivery	7,227,000	2,362,000
Transferable water	8,782,000	9,062,000
Banked water and water for future delivery	55,380,000	41,744,000
Transferable water with AVEK multiple	1.5		1.5
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	43,584,000	33,390,000
Water contracts and purchased water	5,749,000	5,749,000
Transferable water
Banked water and water for future delivery
AVEK water bank	8,782
Transferable water | AVEK
Banked water and water for future delivery
AVEK water bank	3,028
Transferable water	4,542
Transferable water with AVEK multiple	1.5