Earnings per Share/Capital Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Capital stock
Options outstanding (in shares)	0		0	0	0
Issuances of Common Shares under stock-based compensation plans (in shares)				18,351	24,612
Other financing activities				$ (913)	$ (1,236)
Dividends paid				$ 45,294	$ 41,664
Common Stock, Dividends, Per Share, Cash Paid	$ 0.4300		$ 0.3975	$ 1.2250	$ 1.1275
Parent Company
Capital stock
Dividends paid	$ 15,900		$ 14,700	$ 45,300	$ 41,700
GSWC
Capital stock
Issuances of Common Shares under stock-based compensation plans (in shares)		1		1
Other financing activities				$ (835)	(1,134)
Dividends paid	$ 16,000			55,400	$ 27,000
Issuance of common share to parent		$ 10,000		$ 10,000
ASUS
Capital stock
Dividends paid			$ 14,700
Restricted Stock
Capital stock
Restricted stock units outstanding (in shares)	106,879		96,629	106,879	96,629