Long-Term Water Assets - Additional Information (Details)	3 Months Ended
	Mar. 31, 2019 $ / acre ft acre ft	Dec. 31, 2018 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
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	769
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	1,120
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%