FINANCIAL STATEMENT SCHEDULE - Condensed Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 20,686	$ 11,114
Other current assets	8,086	8,593
Total current assets	190,593	190,714
Other assets
Other	35,890	28,893
Total other assets	1,023,426	977,768
Total assets	5,149,566	4,658,309
Stockholders’ equity:
Common stock, $0.001 par value; authorized 70,000,000 shares in 2025 and 2024; issued and outstanding 36,118,242 shares in 2025 and 33,629,169 shares in 2024	36	34
Additional paid-in capital	958,188	827,796
Retained earnings	581,080	537,184
Accumulated other comprehensive income	1,467	1,960
Total stockholders’ equity	1,540,771	1,366,974	$ 1,233,397	$ 1,110,868
Long-term debt, less current portion	1,866,819	1,706,904
Total capitalization	3,407,590	3,073,878
Current liabilities:
Line of credit	86,834	119,124
Accounts payable	75,427	56,256
Accrued interest	18,241	17,476
Other current liabilities	20,942	23,236
Total current liabilities	273,378	261,173
Deferred income taxes	307,893	276,043
Other noncurrent liabilities	23,920	22,298
Total capitalization and liabilities	$ 5,149,566	$ 4,658,309
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	70,000,000	70,000,000
Common stock, shares issued (in shares)	36,118,242	33,629,169
Common stock, shares outstanding (in shares)	36,118,242	33,629,169
H2O America
Assets
Investments in subsidiaries	$ 2,037,519	$ 1,877,922
Current assets:
Cash and cash equivalents	175	1,306
Intercompany receivables	6,680	1,473
Intercompany notes receivable	102,780	74,119
Other current assets	4,372	1,177
Total current assets	114,007	78,075
Other assets
Other	287	222
Total other assets	287	222
Total assets	2,151,813	1,956,219
Stockholders’ equity:
Common stock, $0.001 par value; authorized 70,000,000 shares in 2025 and 2024; issued and outstanding 36,118,242 shares in 2025 and 33,629,169 shares in 2024	36	34
Additional paid-in capital	958,188	827,796
Retained earnings	581,080	537,184
Accumulated other comprehensive income	1,467	1,960
Total stockholders’ equity	1,540,771	1,366,974
Long-term debt, less current portion	557,832	557,430
Total capitalization	2,098,603	1,924,404
Current liabilities:
Line of credit	11,000	3,000
Intercompany payables	0	2,210
Intercompany notes payable	966	0
Accounts payable	3,274	1,588
Accrued interest	3,258	3,224
Income tax payable	31,661	16,871
Other current liabilities	408	765
Total current liabilities	50,567	27,658
Deferred income taxes	815	2,488
Other noncurrent liabilities	1,828	1,669
Total capitalization and liabilities	$ 2,151,813	$ 1,956,219