Earnings per Share/Capital Stock - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Capital stock
Options outstanding (in shares)	0			0		0	0
Sale of stock, number of shares issued in transaction (in shares)	335,449					791,097
Sale of stock, consideration received on transaction	$ 27,000					$ 60,000
Payments for commissions	$ 400					$ 900
Issuances of Common Shares under stock-based compensation plans (in shares)						25,164	18,351
Payment, tax withholding, share-based payment arrangement						$ 1,142	$ 913
Dividends paid						$ 49,384	$ 45,294
Dividends declared per common share (in dollars per share)	$ 0.4655			$ 0.4300		$ 1.3255	$ 1.2250
Issuance of Common Shares from an at-the-market program, net of issuance costs	$ 26,881	$ 16,724	$ 15,584
Parent Company
Capital stock
Dividends paid	17,500			$ 15,900		$ 49,400	$ 45,300
GSWC
Capital stock
Payment, tax withholding, share-based payment arrangement						1,067	835
Dividends paid	$ 17,500			$ 16,000		17,500	$ 55,400
Issuance of Common Shares from an at-the-market program, net of issuance costs (in shares)				1			1
Issuance of Common Shares from an at-the-market program, net of issuance costs				$ 10,000	$ 10,000		$ 10,000
ASUS
Capital stock
Dividends paid						$ 16,000
Restricted Stock
Capital stock
Restricted stock units outstanding (in shares)	135,575			106,879		135,575	106,879