Long-Term Water Assets - Additional Information (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / acre ft acre ft	Dec. 31, 2017 acre ft	Dec. 31, 2013 acre ft	Dec. 31, 2009 acre ft	Dec. 31, 2008 acre ft
Long Lived Assets Held-for-sale [Line Items]
Contract renewal optional term	35 years
Water sold (in acre-feet)	7,442
Water assets sales price | $	$ 7,992
Cost of purchased water | $	$ 3,679
AVEK water bank
Long Lived Assets Held-for-sale [Line Items]
AVEK water bank (in acre-feet)	13,033,000	13,033,000		6,393	8,393
SWP water contracts
Long Lived Assets Held-for-sale [Line Items]
AVEK water for future delivery (in acre-feet)				3,444
DMB
Long Lived Assets Held-for-sale [Line Items]
Contract renewal optional term	35 years
Consumer price per acre-foot | $ / acre ft	738
DMB | Maximum
Long Lived Assets Held-for-sale [Line Items]
Annual fee increase, percent	3.00%
DMB | Transferable water
Long Lived Assets Held-for-sale [Line Items]
Long-term water assets (in acre-feet)	6,693		6,693
PEF | Transferable water | Ranchcorp
Long Lived Assets Held-for-sale [Line Items]
Consumer price per acre-foot | $ / acre ft	1,088
Annual fee increase, percent	3.00%
Water assets, volume available for purchase from 2017-2030 (up to) (in acre-feet)	3,500
Annual option payment, percent	30.00%